First, if you remove the cats, insist on getting them from the mech. They are worth serious $$. They're being stolen all the time.
I would get a second opinion on the cats, unless the shop showed you failure codes condemning them, which I doubt. Also, the Electronic Fuel Injection may not perform correctly without the cats. My neighbor had them pulled off his 4wd pickup, and had to put them back on, so it would run decently.
Throttle body cleaner sounds like something for Throttle Body Injection, which the Windstar doesn't have. In fact, something like that could possibly coat or permanently damage the airflow sensor in the intake.
It's unlikely the wheelchair conversion would damage the exhaust, but a good shop can put it up on the hoist, inspect it, and let you look for yourself.
You have a valuable vehicle there, so it's worth fixing and making reliable once again.
GB | 
