John,
Your budget limits your options, but there's hope.
Consider a Dahon P8; it mght fill your needs.
What and where is your arthritis?
We're the same age. My arthritis is primarily in my thumbs/wrists, but ergonomic bar ends make a huge difference. Flat bars are painful, but moving
to bar ends relieves significant stress. Takes some time to get used to them, since hands there mean a movement to brakes and shifter.
For what it's worth...
Here a P8 is about $1,000....Rheumatoid arthritis and in all joints. One reason for the bike....keep moving.
Whatever bike I get I'll revise the bars and maybe seat for my ergonomics.
Thanks, John