Def go with the model you want right off the bat for MKIV. Get a hard top if it's in the budget...otherwise targa for you. Keep in mind the replacement parts cost assoc with this car. Dealer pricing is higher than a Honda and getting used parts for the MKIV can be diff at times. I have to go nationwide all the time to source used parts... Sometimes hitting up the dealer for connectors, restoration, and maintenance parts etc.
As far as pricing for these they seem to be holding their value pretty good at the moment. I would say they have dropped just a hair in value / pricing vs a few yrs back... but otherwise 20K+ for one in any sort of decent shape.. a TT six speed that is..
Go to the supraforums.com and look at the supras for sale ...you'll get a much better idea what they are worth.