Most people knock the Katana's as being "old-tech" pieces of crap. I say that makes it a perfect bike to start with, given your dirtbike background.
I started riding on an '83 Honda Nighthawk 650, which was relatively heavy for a first bike. It had decent power, too, but I had a lot of respect for the bike's ability to bite me in the ass, and I did just fine. It helped that I was 28 years old, and had gotten most of my testosterone-induced dumb mistakes out of my system.
Having said that, I've got four bits of advice for you:
1. Take the MSF class before you ever put a wheel on the street. It's important, and it should be fairly easy for you to do.
2. Take your time learning to street ride, or you'll be crashing sooner rather than later. DON'T try to "keep up with your friends" until you've got a few thousand street miles under your belt, at least. The biggest danger for you is that your MX background will make you overconfident on the street and get you into trouble. If you can have the patience not to rush the learning curve, then you'll be fine.
3. Buy GOOD gear. Do this before you buy a bike, and before you take the MSF course.
4. Buy a used bike. They've been making Katana's since about 1928 or so, which means that you can buy the same bike as your uncle's without paying for a new one. A 2001 or so should do the trick for you.
Good luck with it, and be sure to post back when you decide what you're going to do.
