I definitely don't recommend an older iPad (2,3,4). Support is either gone, or going away for them, and mine is too slow for FF to be suitable.
The 2017 9.7 inch non-pro is $329 (non-cellular version) and has the newer specs. Plenty good for FF or other software. I went with the iPad mini 4 just due to the size. It works well, and is a notch down from the 2017 model, and a couple notches down from the Pro models. Pro is definitely overkill for FF imo...unless you plan to use it for other things that are heavy tasks.
For just pure convenience, the built in GPS can't be beat, but you'll have to pay for the cellular version.
If you have, or feel, the need for weather or traffic, many external GPS units provide that, and many of those are WAAS GPS as well.
Good luck
Edit: 2017 version on sale now for $299