We had a 4500 sf house but went to an 1100 sf townhouse in between before going FT.
There is a lot to be said for starting with a smaller simpler rig for 6 months to a year and then getting your "real" FT rig. The depreciation won't be that bad and it is a fabulous investment in your education about RVing.
Why not get a TT and pull it with your Expedition to start with. Then you can travel the country checking out all the dealerships and all the models of fifth wheels and visit all the factories and take their tours and meet the folks that build these things.
You can also do a hunt for a truck and interview all the other FT RVers you meet about their trucks and figure out which truck you think is best.
That's what we did, and it worked out great. By the time we bought our trailer we had a stack of fifth wheel brochures that was 3 inches thick.
We got our truck and trailer for a STEAL after a year of really good solid and FUN research while we were on the road testing out the FT lifestyle...