The question says it all. Unfortunately, due to budget constraints and increases in airfare and baggage fees, we're staying off site this time around...
There is no shame in staying offsite! We have never stayed onsite. I think of the $ we have saved over the years!!
Yes you can make your FP selections in advance online.
Bands are not required for FP+. You can just use your RFID-enabled ticket.
Magic Bands are free for on-site Guests. Off-site Guests can purchase them for ~$13. It's mostly just a convenience factor.
If someone buys a ticket at the gate, they can scan it into the MDE phone app and get immediate access to making FP+ reservations (which is pretty much what you'd do if you pre-purchased your tickets and were sitting in you living room linking your ticket, you're just getting access much later). Otherwise, they'd have to wait until they got to a kiosk inside the park to scan the ticket and make their FP+ selections.