Superjet Dan gave a lot of good advice. I've owned a Kaw 440, 550, and an sxr800. I also owned a 94 square nose superjet with a 701 and a 97 round nose superjet. So many awesome skis to own.
When it comes to cheap, you can't beat the fun factor with a nicely modded 440 or 550. Older models are easily found for around $500 typically with a pipe, heads, and some other mods. They're great for seeing if there's an interest - which I can guarantee there will be! Stay away from the 330 (or was it a 340?) as it severely lacks the power with one cylinder. A modded 440 can be a lot of fun, though, and a 550 even more so. Stock I wouldn't look at since you can find a modded one for really the same price.
Stay away from the Kaw 650. I personally think it's like riding a piece of plywood. Although I think the plywood might out corner it! I'm also not a fan of the 750 but that one is more personal opinion. The 650 is more fact haha. The 800 is a great stand up that will excite a lot of new riders since its the easiest to learn on. It's very stable for a stand up and it has a ton of power. But the weight is also there and likes to plow the nose. It's great for high speed and will carve like no other stand up with great power in stock form, but it's not playful due to the weight and it sucks in the surf. They usually start around the $2,500 mark for an older one.
As for Yam... The superjet is King! I never liked the old school square noses. The early models (92 -93) came with a 650cc. Stock form they're dogs. The 94 and 95 models came with a 701cc. Im not sure why I never liked the squares, but in 96 the hull changed to a round nose and the 701cc engine went from a single carb to dual carb. Throw on a pipe, heads, msd enhancer, etc etc etc and the ski really comes alive. I still severely miss my round nose! Superjets are always in high demand so they can fetch a premium. When I sold mine I sold it in one day from listing it and he drove 10 hours to pick it up. I made a $2k profit but I also put a ton of work and labor into it. The FX is also a very fun stand up. It rides like a 550 on steroids. They're difficult to find but every now and then someone doesn't realize what they have and sells them cheap. If you can find one cheap they usually don't last long at all unless it needs a lot of work.
Be prepared to fall in love! Feel free to ask about any you're interested in.