are there any old (i.e. Sero/Legacy compatible) Blackberry models that have WiFi? which models?
Nope. The Only Sprint BBs (CDMA) with WiFi are the new Curve (8530), Bold (9650) and Style (9670). I don't know if the 8350i (Nextel/iDen) is compatible with SERO (it might be), but I don't think it matters since you're probably looking for a CDMA phone.
is the 8330 legacy compatible?
Is that the original curve? If it is, then it works. I have the old curve on a free and clear plan. On another note, if you have sero plan then as of October any sprint phone works for an additional $10/month
I think it is, does blackberry service work on it? does it cost anything extra?
You can only get that with a bb. But it won't let me upgrade to anything higher than the bb tour--but that's not a wifi phone
I don't know if they would offer him a free BB plan. They are trying all kinds of things to get people off these legacy plans.