I wouldnt run anything but premium in it, regardless, the only real way to tell is to listen for pinging or take a look at the piston and see what kind it is. if it is aftermarket, it could possibly need 91 or more. anything less than 11:1 you can pretty much run pump gas on unless it is crappy or has ethanol in it.
the shocks, cant help much there but if they are rubber, they are probably stoppers. they will move and be fine on there, they are only there to absorb the last of the stroke on a hard impact that would bottom it out.
youll also want to check the oil with it warm, after running it at idle, then shut it off. To check the oil level, the engine must be warm, a cold engine will give an inaccurate reading. Let it idle (without goosing the throttle) for about five minutes. Shut the engine off. After a few minutes, remove the dip stick, wipe it clean, put the dip stick back in, but without screwing in any of the threads, then remove it to show the oil level. The oil level should be between the two marks.
sounds like you got a good deal on it!
steve
2019 Maverick X3 XDS Turbo R
2003 TRX 400EX
2016 HONDA RANCHER