[Supras] Old smog machines - what to look for?

[Gary H]

I'm thinking of buying an old smog machine to pretest my cars before 
taking it to the shop for smog testing.
The old smog machines circa 80-90's are relatively cheap now.

Anyone know what to look for when buying these old smog machines? Can 
the old machines measure
NOx (I'm sure they all can measure HC and CO)?

I have absolutely no experience with these machines.
Thanks,
Gary