If you stay offsite for the first few days and you *want* a MagicBand for those days, I believe you will have to purchase it. You will receive a MagicBand for every stay at a Disney resort - so if you do a split stay, you'll get multiple MagicBands (assuming things stay as they currently are, of course. Disney may change things by the time your vacation starts, but you can do just the best you can do with what you have.) But each MagicBand seems to be good only once that reservation has started, so while you may receive one prior to your trip it probably won't be useable until you check into that Disney resort.
Note that the MagicBand is similar in concept to a wallet -- it "contains" the information about your resort reservation, your ticket information, any meal reservations, FastPass+ reservations, etc. But you don't get to make FastPass+ reservations unless you have a ticket to connect to it, you don't get to make an ADR reservation unless you have a room reservation (and, thus, can't attach any meal reservations to the MagicBand unless the room information exists,) etc. All that's to say that the MagicBand is not a necessity - the theme park ticket is what's required for FastPass+ and the room reservation is what's required for the ADRs (at least, to my knowledge, to make them as early as you'd like.)
As far as renting goes, try the rent/trade board here on the forums. There was recently a topic I looked through (though I don't remember which board - not the rent/trade but probably within the
DVC boards) that left me with the impression that 'average' price is around $15 a point, give or take. BWV will *probably* be cheaper than BLT, particularly at that time of the year and definitely if you want MK view. (In fact, good luck getting MK view this close to your vacation -- something like that probably needed to be snapped up right at the 11 month timeframe and you're just a smidge past that.) There are plenty of posts regarding what to look for and how to go about renting though, honestly, the previous poster's recommendation of a broker such as David's sounds like a very good idea for a first time renter. Usually the restrictions on renting that seem to matter to people are a) no changes to dates or rooms allowed, b) no cancellation allowed, c) no 'free dining' and d) daily housekeeping is available for an extra fee (you get housekeeping every 4 days, but your plans are so short for each stay you'd basically get none.) The price is, of course, the biggest pro.
And, yes, you'd get the ADR window timeframe 'as normal' for a Disney reservation. And, yes, it would be considered to be a valid Disney resort stay, so you could make your FastPass+ reservations 60 days early. Just make sure that you tie everything together via the MyDisneyExperience.com website (the MagicBand is basically the mobile version of this website, one that you can carry with you.)