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Just curious how this bike matches up in function, reliabity/durability and especially parts availability vs other smaller KZ street bikes, particularly 440 and 650. It appeals to me more than a 650 because its a few pounds lighter and I don't need the speed/performance of a 650 anyway. But if its a PIA to maintain and find parts for maybe I should stay away. I know it won't be as easy to find parts for as a 650, but is it hard? I know it is getting hard to find parts for 440s, which would be my first choice if not for that fact.

And what's one worth with 15K miles, that seems to be in good running order?

Is it hard to find parts for 440's?
I don't think you can compare a KZ440 to a GPZ550 or a KZ650, 4 cylinders are a completely different ride experience.

I rode a KZ440LTD belt around for a few days one time when my KZ650 was in the shop.

I think they sold less 550's than 650's for sure.

KZ650 ran from 1977 to 1983
KZ/GPZ550 from 1980 to 1983

I have no issue finding parts for my 650's, some parts are reproduced (some shared with KZ900) of course vintage Kawasaki replacement parts are always $$, some parts are NLA to buy from the dealer but you would be surprised what you can order.
 

It's not as hard finding parts for 440s as it is for my 305, but it does seem to be getting pretty hard. I don't have a 440 but that fact has so far kept me from getting one. I don't want to find myself in the same boat I'm already in. I already have that boat. But I really prefer a twin, just doesn't seem sensible anymore.

Edit: Oh and the KZ550 ran through 1985

In my experience, 440 availability isn't actually bad - it's just inconsistent.

Pre-pandemic, I wanted a KZ440B but couldn't find one for a year or two so I bought a 440LTD to modify. The following year (I think) I found a B complete basket case and a B rolling frame - with title! - that I could put together into one road-legal B. It's getting VM30s and a 7th Gear electronic ignition, so that eliminates the more serious OEM parts rarity.

Obviously I'm not worried about keeping it stock, but if you want ride a 440 you shouldn't have much trouble. If you want to restore a 440, that will be more painful.

I would look into parts that are most likely to be hard to find, things like valve shims, carburetor parts, gaskets, air filters. 
The KZ550s have Teikei carburetors, and the motorcycle stuff page by loudhvx (deceased)  has lots of Teikei & 550 info.
You could see what Z1Enterprisess.com has; choose a 550 model then go from there. 
Drive chains, bearings, oil filters & tires shouldn't be a problem. 
Or just look into a more modern motorcycle? Something that's 10-20 yrs old instead of 40+. hee.
 

Taking martin-crs point a little further.. If parts availability is your concern, which is a good one, then why not move to a more modern bike? 
I suggest the Kawasaki zr7 or the fancier zr7S sold in the US between 99- 03. It is an air/oil cooled 750 inline four with about 65 rwhp. It’s no powerhouse but it’s comfortable to ride, reliable and easy to maintain with a more modern frame and steering head angle. There’s no fancy electronics and parts are still available through dealers.
These are old school bikes that Kawasaki decided to build using a then twenty year old engine during a power/performance war between the manufacturers so many people walked right by them in the showroom.
Really nice condition 7's can be had for a couple grand or less. Fixer uppers can be had for cheap. I bought mine as a winter project and refurbished it so it runs like new.

My 7S got the zr7S fairing buzz at the headlight, which I sorted out, but every now and then I get hankering to convert it to the round headlight style of the zr7. But I get lots of compliments about how nice a motorcycle it is from other motorcyclists as they recognize it as an "old" bike. I’ve owned it now for over two years and on a recent ride I had to pull off in the woods to take a leak. On the way back to the bike, parked on a dirt path surrounded by trees with the sun shining on it, I thought to myself.. now that’s a nice bike. I’m keeping it.

The zr7/zr7S is an anarchism worth considering.

I can't afford a modern bike, and also don't really like them. I also don't want or need a larger bike. I'm really pushing it even with a 550 or 650. I've been looking for a 440 but in 2 years have found nothing worth buying except for one that came up when I was out of town for several months, so I couldn't even look at it. Damn near bought it anyway but probably smart that I didn't. And I've nboticed a lot of people having hard time finding some parts for them over the last 2 years. Although that's my ideal size.

A 550 came up which is why I asked, but I'm waiting to learn more about them and for the price to come down before I would actually get interested. I still have a free 650 waiting for me but its 450 miles away and has been sitting in a warehouse for 30+ years with the carbs off.

In 2016 I bought an 81 KZ550 for my wife.  We rebuilt it over a winter and she rode it for three years.  I really liked it.  Lots of power, six speed tranny, very good on gas.  It's a bit small for me, but I'm 6 ft 3 in.  If you're my size you won't enjoy the seating position because your knees will be high.  We had a couple of issues with the TK carbs, but they seemed well matched to the power band.  You can cruise all day at 70+. mph and the bike won't really be working hard.

A KZ 650 is a more powerful bike.  Also a bit heavier I think.  I'm re-building one this winter but I doubt that I'll ride it much, if at all.  I prefer my KZ1000 and my Z1B.  

Ok, happy hunting. My point was there’s nothing modern about a zr7. It has the same design, engine and electrics of an 80’s bike, only more recent. And it would probably cost less to get it to a certain condition, if it’s not already there, than an eighties bike. It’s easy to buy an old junker then throw two grand at it.
The free 650 seems the way to go.

wrote:

In 2016 I bought an 81 KZ550 for my wife.  We rebuilt it over a winter and she rode it for three years.  I really liked it.  Lots of power, six speed tranny, very good on gas.  It's a bit small for me, but I'm 6 ft 3 in.  If you're my size you won't enjoy the seating position because your knees will be high.  We had a couple of issues with the TK carbs, but they seemed well matched to the power band.  You can cruise all day at 70+. mph and the bike won't really be working hard.

A KZ 650 is a more powerful bike.  Also a bit heavier I think.  I'm re-building one this winter but I doubt that I'll ride it much, if at all.  I prefer my KZ1000 and my Z1B.  
 

I'm 5' 7".  Did you have any trouble getting parts for the rebuild?

wrote:

Ok, happy hunting. My point was there’s nothing modern about a zr7. It has the same design, engine and electrics of an 80’s bike, only more recent. And it would probably cost less to get it to a certain condition, if it’s not already there, than an eighties bike. It’s easy to buy an old junker then throw two grand at it.
The free 650 seems the way to go.
 

I do appreciate the info, it expanded my knowledge of Kawasaki bikes. I'd take on if it was given to me but that particular bike is fugly though. To me nothing matches the classic look of the old KZ bikes.

As far as parts availability goes, I would suspect the 650 is the best due to sharing a lot of 750 parts, the 550 is next but the carbs are a bit of a nuisance and the 440 is last due to lower production numbers. As for rideability, I'd be all over a 650 or a 550 (esp. the GPz). The 440? Meh. I have a KZ400 and it's a cute, fun little commuter bike with front brake issues due to NOLA parts.