I've worked on a lot of older carbureted Seadoos up to 1999. Past that is when they went to fuel injection and four strokes. Winterized/summarized them and other makes, but pretty much know older Seadoos. 92' and up are good reliable skis and easy to work on. Engine evolution: 587(580), 657(650), 657x(650), 717(720), 787x(800), 787(800) 110hp, 947(950) 130hp, RFI, DI and then four strokes. Look at the engine plate for cc size, it will have the first number. 587, 717, 787 for example. Stay away form engines painted yellow(very first engines), 787x(produced one year 1995), White 947, and 97'-98' silver 947(had water ingestion and pump issues). 97'-98 silver 947's were updated to fix the engine water ingestion problems, alot of engines went down and you might by one of them. 947's are harder to work on then the earlier engines, because it is shoehorned in the hull. Depending on if it is a single/dual carb, a 1, 2, or 3 seater a 580 might go 30mph, 650/720 might go 40mph, 800 50mph, and a 950 60mph. The bigger the ski and sponsons, the more stable in the water it is. Being stable shortens the learning curve, helps you stay on, and makes it easier to get back on. Seadoo sold 1, 2, and 3 seaters. Once you learn to ride it, the fun factor goes down with the more passengers it will carry. Although some three seaters can pull a skier. One hull to pass up is 97'-98' xp's, maybe all years. They were more difficult to work on and had major issues with the hull ingesting water, then into the engine. Some of the fixes then were rubbing grease on the seals for hood/seat, and a bilge pump. Might be fixed after those years. They sold that hull until 02'. It had a shock under the seat. Call me if you find a Doo and have questions. At the right price I wouldn't be afraid of buying one with a bad engine. Here is a good place for inexpensive and warrantied long blocks. http://www.shopsbt.com/jet-skis/sead...-standard.html
Stand ups have a bigger learning curve. I don't think they offer a relaxing ride when wanted, like sit downs do. Although with a good rider on them they look like a lot of fun. I tried riding a stand up once. The pump wash almost pushed my shorts off as I was trying to get on while starting to go. I got it to stand up for 75 yards or so, with turns. It wore me out.