Are there any prob buying pressure washers from pawn shops

First things to check is are air filter and exhaust. If there is signs of oil then the unit has been tipped and you will probably need to strip the cylinder head to sort it. Also check engine and pump oil levels and start the machine if you are allowed to. If it starts after a few pulls then you have a good chance of it being ok. White exhaust smoke is bad if constant as it is oil burning. Blue smoke isn't quite so bad as it indicates (probably) rich fuel/air mixture. Puff of white smoke that doesn't stay constant is also a sign that the machine may have been tipped at some time and it is just the oil residue burning of the cylinder head.