You'd probably want to have a lead to a tuner before you go purchase the PSC1 unit... If you want to tackle the task yourself, you're going to chuck out about another $500 on the wideband which is needed to tune your PSC1...
Not to mention a bit of welding for the bung...
Then a laptop to run the software off...
A tad more work than a set of rims would run you

I'm shocked no one has any offsets for you yet... I don't have rims and haven't really found any offset #'s for 20"s... I'll see if any comes up later...