All of the Rotax 2-stroke engines are good, but I would personally avoid the fuel injected versions. They are good... but when you have a problem, it's harder to figure out whats wrong, and it can get expensive with just a basic issue. (+$300 for a fuel pump)
Positives...
1) The smaller the engine, the less fuel/oil it will burn.
2) The smaller the engine... the longer they live. (I have a 20 year old 587 that runs perfect)
3) bigger is faster.
Just think about what you want to do... and buy accordingly. The 800 engine is a good compromise with size, and power... but it has a few things that make it a little higher maintenance than the smaller engines. I personally like the 720 engine for the absolute compromise of power, reliability, and longevity.
For family fun, get a 3 seater with a 720 engine. Depending on the year... it will be a GTi or a GTX.