How to solve problems in new kz400d purchase

Hello to everybody, my name is Fabio and I write from Italy. I'm planning to buy a z400 with th famous oil leaking problem from the valve cover. I would like to know if replacing the gasket with new ones as I already read in this forum will solve definitely the problem or if it will start again after some time/miles.
Another question: even with hot temperatures and no choke the bike takes a while (5 minutes) to keep running without a little throttle opened by the rider. Is it normal? Any suggestion about this thing?

Thank you in advance

PS: sorry for my english, I hope you'll understand the questions

Can't guarantee anything but there are plenty of 400s out there that Don't have leaking oil problems so make yours one of those.

The rough running at startup is symptomatic of dirty or mal-adjusted carbs. When the engine heats up enough that aids in the further atomization of the fuel mix. Try added a little fuel system cleaner like gum-out if you don't want to dig in right now. You'll likely see an improvement in short order after a little riding.

Welcome to the forum.

See if you can find me a job over there will ya? Where abouts are you?

Thanks for your answer! I just wonder if recent gasket would have solved once for all the problem but I guess it is a problem about the engine itself.
If I will buy the bike I will check carbs and clean them.