The Transits are already being converted for handicap accessibility. Unfortunately, they’re only being done with a rear-entry conversion, which means the wheelchair guy sits behind the back seats, and has someone drive him around.
By far the most popular minivan to convert is the Caravan/T&C. It’s the least expensive, and the most roomy for a wheelchair conversion. That said, if you don’t know for sure that you’ll need the whole conversion, buying a van with that in mind now is pointless - most conversion companies will not convert a van that’s more than a few years old because they don’t want to be liable for structural problems that may or may not be their fault. So, buy the vehicle that works for you now (the transit might actually fit that bill) and keep the full conversion options in mind in case you need them in the future.