1. No you cannot use an "unlocked" Android or iPhone on Boost Mobile. The iPhone is not allowed on the Boost network at all (at least not yet), and only Android phones whose ESN's are on their list of "permitted phones" can be activated. There is a way to get unauthorized Android phones on Boost, but that involves *questionable* ESN alteration, and I don't think that is what you meant by "unlocked."
2. There is no Pay-As-You-Go (Paygo) plan for Androids on Boost Mobile. The only plans offered for Android phones are the $55/month Unlimited Everything plan, and the $2.00/day unlimited plan. The $55/month plan reduces itself to $40/month after 18 months due to Boost's Shrinking Payments Plan, however the $2.00/Daily price does not change.
3. Most, if not all, Android phones have WiFi, so that really isn't a problem. Since you are new to Boost, I recommend you purchase an "authorized" Android phone from Boost or eBay or wherever, as you will be able to activate it without any hassles. Also, if you get the new HTC EVO Design, your plan will include 4G (where available) at no additional cost. Meanwhile if you try to use a "modified" Android phone, you will be guaranteed to not get 4G service, even if the phone is capable of it.
* Incidentally, all of your questions could have been answered by looking at the BoostMobile.com website.