First things first, Where are you located?
$1,400 for a banshee that needs a top end, is way over priced, especially for an essentially stock bike. Reconsider the price. I've seen complete bikes that need top ends go for as little as $700. Again, it's going to vary depending on where you're located. If you DO end up buying this bike, there are several site sponsors on here who do amazing work, and have awesome prices. Wilcard, and Herr Jugs Racing are just a couple.
There could be several reasons the bike is hard to start, if the top end is fine. DO A COMPRESSION CHECK prior to buying the bike. Someone could correct me on this, but I think a stock motor should be roughly 150-170psi per cylinder.
If the guy is saying "it doesn't run right in the winter", and you're buying it in the winter, be VERY cautious of this guy.It is NOT normal for a banshee to not run right in the winter. With proper jetting, the bike should run perfect no matter what the temperature outside is. If it's hard to start, and runs terrible when it's cold outside, it sounds like it might just be a carb problem, rather than a top end. But the only way to tell is to do the compression check. Go buy a tester if you don't have one, you'll use it a bunch.
In conclusion, just be very cautious of this bike, it sounds like it might be full of headaches. I hate to see new banshee owners on here complaining right after they bought a bike, that it's a pile of poo and they have to immediately dump a ton of money into it. You did the right thing, you came here first!!