As for the size, the smaller ones most times have a corner bed,which may be fine for you.
The high mileage is not a problem if it has been well maintained and documented.I would be weary of one that is old and has low mileage,that means it has sat a lot and these things must be used.
As for buying from a rental agency it is a real good deal,they have to keep up the maintenance on them and keep a record of it and you will get it when you buy one.
You can do some "light" reading about them in the
Clicky in my signature. You can see what you get as far as records and tires and such, they are ready to go with out having to put money into them to get them road worthy. They also come with a warranty.
The mileage between the sizes is not much different. Many people tow behind small RVs without a problem.
If you want to look at a Rental in your price range and in Florida
there is
This One