check engine compression and hold throttle wide open while testing, should be at least 130 and both cyls near equal. listen for a slight ticking sound in the top end, this is known as piston slap and occurs when it ingests water. it is more prevelent in the xpl models but can occur when there is a faulty pipe seal which is a common problem on the 951. if the tick is strong, it will need a top end job, minimum $300 if you do it yourself.
definitely water test it and check the hours and the function of the VTS and fuel gauge. both simple fixes but may give you bargaining material to drop the price a couple hundred.
check to make sure the head has never been off or ask why it was, the silver factory paint will be heavy and the head gasket will be painted silver as well.
I think the 99 gsxl is one of seadoo best ever models and will keep up with an rxp, it should hit 70 on the speedo on nice water. that's a good price, I sold one last year with 90 hrs for $2300 with a trailer and cover which was a good price.