200tq it is(was nitrous/turbo)

[Festively Jolly]

   Well, it looks as though i am going to purchase a 1989 200 tq and
sell the 90.  Now, i am not familiar with these cars.  Anything i should
i should keep in mind while purchasing it?    How much should i spend on
one with about 100,000 miles?  Now, I am not educated in the ways of the
turbo. I don't eXactly understand the boost situation.  Is it
adjustable?  If so what the ideal psi should it be set at?
THANK YOU VERY much for all of the info so far.  thanks in advance for
any more to come!