pull the floor boards and check the insulation between the stringers, if any (US boat, so may not have to meet the unsinkable reg that Canada has for smaller boats). If waterlogged it adds a TON of weight and greatly affects performance with a jet. Floor boards should be easy to pull as anyone owning a smaller boat that has to meet unsinkable regs should pulls theirs regularly to ensure insulation isn't sucking up water. Newer foam apparently doesn't have such a bad problem but stuff used in quite a number of manufacturers through 90's and early 00's sponge up a surprising amount. Also, depending on how sturdy you want your boat to be number of stringers matters if you're likely to be bouncing it off some rocks in skinny water (e.g. hunting up the Gateau or Tuchodi). For those applications the likes of Alicraft and Outlaw have 9+ stringers in the bottom. If you're fishing deeper/milder water not an issue and lighter may be better for performance and fuel $ (jets are thirsty!!). Similarly, if running skinny water a bottom coating of "teflon" (UHM or whatever the acronym is for the plastic) is a big plus as aluminum grabs rock and gravel. Again, if not needed you won't want the extra weight.
Get him to pull the impeller and sleeve and look for scour and wear from sucking up sand and gravel. Doesn't take very much wear to cause cavitation (i.e. blowing air bubbles along with water = reduced power), which greatly affects performance of your jet. Owning a jet will mean you'll go through sleeves and impellers, though - nature of the beast. Of course, no substitute for running it as height of intake, dents on bottom, etc, etc can cause cavitation and performance issues. A bit of lag to get up on step isn't the biggest concern as a cheap fix of welding on a couple of trim plates often makes a world of difference for very little $'s. Lower units have to be well greased and should get a shot in the nipple until clean grease comes out before every use. Get him to do this and look at what comes out. A bit of off colour from sitting and the last use isn't bad but if it looks like the grease hasn't been changed or maintained regularly the motor's likely been abused and not maintained well. Any way, a bit of a list of some things to consider/check. Far from comprehensive.
Ukee