I have supported my wife's use of mobility scooters for the last 24 years by trying every different arrangement of lift and scooter size. We traveled consistently in that period including living in Malaysia, Kuwait and Puerto Rico. Some thoughts
For use in house, stores, restaurants, developed areas, three wheel smaller is better Travel scooters define the application. Ground clearance and tire size limits off pavement use.
Used a midsized 4 wheel for about an hour before taking it back way to large a turn radius
Used a midsized 3 wheel for 7-8 years. Gotta have a vehicle hoist because of disassemble complexity and weight of parts. Ramp is dangerous, you will drive it off and have it fall on you eventually. Ground clearance is good and tire size helps use off pavement.
Define your requirements based on where you will spend 95 % of your time. Ie smaller is better except when offroading. Do you really do much off roading.
A travel scooter is light enough to lift in and out of pickup if you can pick up 40 pounds. I do have a lift in my Toyota Seinna because it is easier. I do not use a lift in the toad or rv.
Pass on insurance and medicare to pay for it. The hassle is not worth it. The supplier will work off list price and ins cover will have deductables copays and in net work requirements.
Use Spinlife . com as your supplier. Pricing is wholesale about half the list and they will deliver to your house. Set up is no more difficult than take down to put in your truck/car each day.
The most expensive cost of owning a mobility scooter is the improved ability of your spouse to shop.