Hi,
And welcome to the forum. Here is a great info page for SeaDoo watercraft since they came out in 1988 through 2010.
Sea Doo Model Reference (1988-2010)
All the "G" skis you mention are all three seaters.
GTI is the entry level model with the least powerful engine
GTS is the middle of the line machine
GTX was the flagship until the RXT came out.
If you are looking to get a 2 stroke the 800cc engine was SeaDoo's best engine reliability wise. The 951 had more power but were less reliable. So you have to decide what's your priority. Otherwise look for a 4 tec four stroke which came out for the 2002 model year.
2 strokes usually last around 200 hours but I've seen some go for much longer. There are just too many factors involved to pinpoint an exact boundary when a unit has "too many" hours. Be sure to do a compression check before you buy.