Approach any potential car the same way - eyes open wide.
Ex rentals are very likely imho to have been driven hard. I certainly give any rentacar a bit of grief!
Check the following very carefully:-
Bodywork. As the dealer has the car been in an accident, or had any bodywork done.
Suspension/wheels/tyres. Listen for any knocks or unusual noises. Regular lhd pilots regularly damage the lhs of the car against kerbs and ditches. Look underneath too.
Gearbox and clutch. Texas Ted may have done a few dodgy gearchanges and ridden the clutch.
An 04 rentacar with 8k is fairly low mileage, and you are right to be suspicious, but it may turn out to be kosher.
Good luck.