Welcome! Welcome!
Have you ridden a motorcycle at all prior to this 'interest in MX?'
In general, dirtbikes have a few points that you want to look at before you buy. I'll hit on a few of them, and others will chime in with excellent information as well.
-Fork seals leaking?
-Engine leaking any fluid?
-Tire condition? They are nearly $150 for a pair.
-Tubes holding air?
-Is the chain lubed and adjusted? If it's dry as a bone or rusty, that chain was neglected.
-Look at the air filter. If it's filthy, then it has probably never been cleaned, and I doubt the oil has been maintained.
-Lift the bike (from the rear fender) until the weight is off the rear suspension. Is there any slop between that point, and when the rear tire begins to lift off the ground? Rear linkage can be VERY expensive and difficult to change by yourself.
-You can usually see brake pad thickness externally on a dirtbike. anything more than 2mms and it's within reason.
Have them start it. If it doesn't start in 5-6 kicks, it needs something adjusted or fixed.
-BIG DAN