I started on a 98 750 XI before I discovered stand ups. I paid $700 for it with trailer and sold it without the trailer for a bit more. They are plentiful and parts readily available. Power output is decent and rides pretty good.
I think a faster ski would be a early 90's Seadoo XP with that powervalve motor. A little bit more expensive than a 750 XI though. You can tell there's better hardware in there just by the huge expansion chamber they include with it. They're not bottlenecked like most OEM skis. I think they may be a little less reliable than a kawi or yamaha, but Seadoo makes it up in power and ride (they are supposedly pretty good in the surf, and I actually liked the way they ride more than the 750 XI)
Another ski I owned was a 1999 Yamaha GP760. It's a newer styled ski I was able to find for $500 with a blown motor. I put a motor in from ebay for $700. That ski was a little larger and less playful then the previously listed, smaller skis. It definately makes up in the chop though. I had 2 motor mount inserts pop out of the stringers, but easy to repair. Nothing fast or amazing about that ski, besides being great for passengers, great on fuel and having a nice ride. Damn thing had a 13 gallon gas tank! Maybe one of these would be best since you'd most likely already have spare parts. EDIT: Almost forgot, the GP hulls porpoise pretty bad over 45 mph. Trim tabs supposedly help a ton, riva makes them and are discontinued.
Fortunately, sit down skis are plentiful and there's usually good deals out there that you can't be hurt on. I usually choose a price to cover my ass. If the engine blows next week and I have to replace, I still don't get hurt on the deal. Nearly impossible to do with stand up jet skis since they're usually price gouged. I'd use that to your advantage. I only loose money on stand up skis haha