All i respond to is quads. Lets start by throwing out the saying “adult driven”. Its really almost a joke these days. Im a adult and I beat on my quads. Thats why they where made Here are some things to look for. Pull the seat check to see if the wiring is hacked up, check the air cleaner, oil, differentals see if oil is “milky”. Take it for a ride make sure choke, lights, everything works. See if it smokes when you hammer on it. Ask if you can jack it up and look at the wheel play(bearings,tie rods and ball joints). If its got oversized tires on it you can bet one of those is bad. I like a v-twin in anything over a 500cc i dont think the motor has to work as hard as a one lunger with the extra torque. Just my opinion! I have two that have 27″ tires on and programmers and they run great. I have to replace the ball joints every year and check my belt. Thats another thing if possible look at the belt bigger tires are a little hard on them. Basically whatever you are looking for research it and see what its known for for break downs. I just sold a 06′ with 1400 miles on it last year and I did’nt think the milage was that bad on it. Any other ?’s just PM me I have a few toys and I work on them to so maybe I can help. Hope everyones replies help.