Hello, wondering if I made a mistake.

Hello y’all, I just recently purchased an 81 750LTD for 150 dollars. I’ve wanted a bike for a while, and figured I’d get one to have as transportation for college. 

Previous owner let me know that it hasn’t been ran since 2015. I am aware that on Bikes this bad. Anyone have any tips on where I should start on it in terms of actually getting it started?

Previous owner cleaned the carbs and put new seals in them, but didn’t synch them, and I know I need to clean the fuel system and get a new battery. but is there anything I’m missing about what it needs after sitting that long?

Welcome to KZRider
Buy or download a manual for your bike 
Go back thru the carbs  and add fresh fuel and battery
 See if it will run , then do a complete tuneup
Dave

You will need to look at your brakes.  You can't go unless you can stop.  Check the manufacture date on your tires.  

There should be a factory service manual in the Filebase here on KZR. yellow/gold cover with color wiring diagrams. There's also a poorly copied version floating around the internet, so be sure to get the good one.  The diagrams at Partzilla are handy.

I would remove the spark plugs and put a small amount of oil in there or spray some WD-40 into the cylinders.  kind of spray around. then reinstall the spark plugs. you could check the gap while they're out.   Then remove the RH engine cover. use an impact driver on the engine cover screws (I use a hand impact driver). remove and service the timing advancer. turn the crankshaft using a 17mmm wrench on the big "nut" to lubricate the cylinder walls. I would go ahead and get a new gasket. I keep several on hand. make sure the engine oil is between the lines at the sight glass.
warning: from the manual. do not use a 14mm wrench on the timing advancer bolt to turn the crankshaft >> doing so could damage the advancer.  

New battery. the motorcycle needs a good fully charged battery to start and run right. never assume a battery is good. I use a Ctek US 0.8 smart charger (~$50) and a VC97 digital multimeter (~$30). you could spend less. or more. ha ha.

Drain the fuel tank and carburetors and add fresh fuel. try using the old fuel in a lawn mower mixed w some fresh. You might to need to clean the carburetors. However, before going to all the trouble, you could do the other stuff and see if the motorcycle will start.  But if you've never done carb work before, it's not that hard. most stuff is done 4 times. try to return the little parts to the same carburetor, instead of mixing them up.  use good fitting tools. you could practice on one carb, remove as much as possible, then reassemble.  There may be a simple mechanical synchronization you could do.

Check the air filter and clean out the airbox if necessary. 
I would go ahead and change the oil and oil filter. Shell Rotella 15W-40 regular is typically cheaper than other oils. You should at least remove the oil filler cap and sniff for gasoline. you could fashion a dipstick to retrieve a small amount of oil to check it. to see if it's a nice amber color, or black or whatever.

Just a pic under the RH engine cover. the KZ750H Ltd is the same. your advancer should have a different number stamped into it. 1013 I think.

^^ What martin_csr said.
I would add that you should make it safe and reliable first, then work on the appearance items. You'll probably want to check valve clearances too. What shape are the chain and sprockets in?
In the meantime, let's see some pics. 

I'll add before buying a battery. You could try starting the motorcycle using a 12VDC car battery (car engine OFF. car keys in pocket. ha ha). Just connect jumper cables between the motorcycle and car.  This is what I did and I removed the old motorcycle battery since it was bad. The motorcycle doesn't care if the battery is too big, just so long as it's 12VDC and good. I would never jumper a motorcycle with the car running. plus the mc starter motor is so small it shouldn't be necessary anyway. make sure the jumper cables are secure and can't be knocked off >> you don't want to cause a short. and don't connect the cables backwards.