[Miyamoto]

I've had a Kawasaki Ninja 500 and I ride a Katana 600 right now.
Available parts are plentiful and fairly inexpensive. Its "better" looking than the Kawi (my opinion) and you're right, the Kat is a comfier sit.
And...if your @$$ is really 320lbs you have NO business looking to get on a 250cc bike anyhow (that's just asking for trouble in my humble opinion).
If the Kat is comfy for you right now then that's the most important thing for a beginner. In fact, for all riders, because trust me, if you're not comfy on the bike, you lose focus, your mind starts to wander and do other funny things that lead to bad judgment on the road.
Take the course, get the bike, be comfortable on it as you get USED to it.
Performance-wise, its a powerful enough bike. You won't outrun any Gixxers (at least, not until you've ramped up some experience points), but that's not what you're getting the bike for anyway.
As you feel more comfortable, you can always jet the engine and install a performance pipe for more zip.
Ride Smart.