We are dedicated offsite people, we love it. First, I would very much disagree with the previous post. Traffic really isn't a big deal at all. Sure, there are cars at traffic lights, but it's not a madhouse rush hour like you'd see in NYC or Atlanta or LA. It's nothing to be concerned with, IMO. There are a lot of tourists of course, so be aware of people who don't know where they're going, but I don't think drivers there are crazy by any means. In fact, it's the opposite, IMO.
As for where to stay offsite, there are endless choices. VRBO is a great place to start looking. You can get a 3-4 bedroom townhouse or condo for $100/night all in. The area I'd suggest looking at is Rt. 192. There are dozens and dozens, if not hundreds, of communities along this road. It's a short drive to WDW and is full of restaurants, gift shops, etc... Most of the communities are gated, with a clubhouse, pool, amenities, etc... We like to rent from private owners rather than management companies, it gives a more personal touch.
As for how FP+ works, since you'll be offsite you'll have a 30 day window. But not to worry, you'll still be able to get FP+. There is a FAQ sticky in the Theme Park section of this board that goes rather in depth about how FP+ works and how to best use it. Read it, learn it. It'll be one of the keys to getting on lots of attractions.
I think you may be asking about Extra Magic Hours? Those are for onsite guests, but honestly you aren't missing anything...except some sleep. We've done one or two, and they were nothing special. Again, learn how to use the FP+ system.
How may days do you need? Well, that depends on your goal. If you've never been and are taking a typical one week trip, you will barely scratch the surface of what WDW has to offer. There is just so much. We're park veterans and from 2015-2018 spent 100 total days in the parks (plus all the time on previous trips) and there are still new things we're checking out....though some things we just never wanted to until recently. My suggestion is to spend time in each park and don't worry about trying to see it all, because you won't. Just enjoy what you do see and take the time to "smell the roses".