The FX HO is Yamaha's upper-mid range 4 stroke ski that has been around since 2004. The 04-07 models are basically the same ski with a 160 hp engine and they will run around 58-60 mph stock. There are 2 models (the FX HO and the FX HO Cruiser). The Cruiser has the seats with back rests, pop-up cleats, and a few additional features. In 08 Yamaha changed the hull material (Nanocel) but the rest of the ski remained the same, and in 09 they changed the engine to 1800cc, so the 09 and 10 models are a new beast entirely.
The FX HO is a very reliable ski, and very stable in rough water. An all around great family machine. Great for towing, multiple riders, dry ride, fuel efficient (also runs on regular gas). The FX HO has a trim adjustment on the left handle bar, and adjustable handlebars that are nice if you ride standing up or with passengers. It also has reverse, and lots of storage space. There are several neat gismos on the display (fuel management, compass, water and air temp, trip meters, etc.)
Outlaw has already covered the stuff you need to look at. I would add to take a close look with a flashlight at the impeller and how closely it fits (there should be no gaps or dents in the impeller or housing). On this particular model you can't check the oil level until the engine has been thoroughly warmed up since it has a dry sump system. If you check the oil cold it will always read too low. I would recommend pulling off the air filter cover (2 screws and 4 slide clamps) and check the air filter, which should be clean and completely dry..........no oil. One very common problem with this model ski is overfilling the oil system, which in turn causes oil to overflow into the air filter, which will significantly reduce performance, foul the spark plugs, etc..
How much salt water use the ski has had is also something important to ask, as well as to whether it was flushed after each use or not. If it has been in salt, take a real close look inside the engine compartment for signs of corrosion.
I second Outlaw's advice on taking a test ride. Some skis may start and run fine on the hose, but this would not show a problem with the pump system. Some sellers do not like to let a potential buyer (especially someone new to jet skis) take a test drive. The seller may be willing to drive the ski with you as a passenger or do a demo ride while you watch. If not, you can write up a conditional bill of sale that would allow you to return the ski if there was a problem when you got it in the water. I will add that problems with this particular model are very rare.
If you do end up getting the skis, on this particular model, Yamaha has issued a "code 90" for the hoses and clamps, since there is a potential for them to come loose. The "code 90" is not a recall, but very close to one. If it hasn't already been done, you can take the ski to any dealership and they will replace the hoses and clamps on the ski at no charge. The owner may or may not know about the "code 90" since Yamaha does not notify owners, but only replaces the hoses when the ski shows up at a dealership, but it is something I'd ask about to see if it has already been done or not. You can call a dealership with the HIN # to see if the ski you are looking at is affected, and needs the hose replacement.
You can download an owner's manual for this ski here:
My Yamaha Prompt - Owners Manual
Good luck!