Huh. I will try to avoid any brand discussion and stick to the normal stuff. I will tell you that I would not buy a USED RZR, too many that people let sand/dirt into them. I changed the air filter design before it touched dirt. That said the Polaris is the performance winner every day.
1. Be aware that an attempt has been made to outlaw the 4 seat conversion in CA. and may or may not be enforced where you ride. If you want one that holds 4 people buy one that was made that way, a number of MFR's make them.
2. On an 05-07 I would be worried about total miles and how much dirt the thing has eaten. Ask about air filter maintenance, you want to hear that they maintained that frequently.
3. Spare belts are always good to have, so is a simple tire repair kit. I have full spares in the bottom of the trailer storage, on the trail a repair kit should be enough unless you are REALLY far from camp. Some basic tools are useful on the trail.
4. 4 or 5 point harnesses are a must. The MFR's don't put them in but I would not ride without one, I have met people who are no longer with us for lack of a harness.
5. In CA. helmets are the law so that won't play into the discussion.
6. Check the dedicated forums, I could point you to the RZR one but there is one for every brand.
7. I would recommend long travel for the deserts, especially on a used machine.
8. I'm pretty sure those years not fuel injected which makes them a little cranky in the cold. Less of an issue in the low desert but it can make them harder to start in the high desert.
Good Luck!