I'm with super66 on this one. Buy some used 17's (typically I've seen them around the $400-$500 range) and put some 17" rubber on them. May want to price out the cost of 17" vs. 18" rubber as in many cases there is a dramatic difference) and may easily justify the cost of the wheels (did so on my track wheel/tire setup). With the stock 17's from an early model Boxster S you should have 7" width up front and 8.5" in the rear which you should be able to get some much narrower rubber for. Depending on the tire (go to the manufacturer's website and they should have a chart of wheel width for a specific tire size), I think you should easily be able to get 205's on the front and 245's on the rear with this set up. May be able to go even narrower?
BTW - super66, djomlas, and mylamb, Are you going to join up with myself, NickCats and bmusatti for some autocrossing next year?
