Here are some of my thoughts.
No matter what you do, get the wheelchair first. Then get a van into which the wheelchair will readily fit. Don't rely on specifications, as they can be misleading. Though they may indicate a particular wheelchair and patient will fit, it may not be a fit that is practical for regular use. If at all possible, test your wheelchair in the van with your mother before you buy the van.
A wheelchair will eventually be necessary. We will all lose our ability to transfer. So, if you get a solution that does not have your mother riding in the wheelchair, consider it a temporary solution.
If she is going to be riding in the wheelchair in the van, recognize that it is quite valuable to be able to see out the windshield/windows. This is not likely to be the case with a full size van where your mother sits in the middle row (not all full-size van conversions require this).
Side entry vs. rear entry have benefits and drawbacks. We chose side entry and are glad we did. Before doing so, we toured the parking lots we are likely to frequent and confirmed they had plenty of van accessible spaces where I could load and unload from the side.
I favor a solution that has the least practical modification to the stock vehicle. With a minivan, new suspension is added, the floor is lowered, and the ramp is installed. I was concerned all of this could compromise the durability and reliability of the van. That could be a concern if you are buying a well-used van.
Wheelchair vans are not the only option. Be sure to consider public transportation options. We have a bus stop a block from our house and all the buses accommodate my wheelchair. Riding the bus is GREAT!
We wound up with a well-used 2004 Chevrolet Express 1500 full-size conversion van with a Braun Vangater II lift. The van has an EZ Lock system to lock the wheelchair in place. While there is a seatbelt for the wheelchair passenger, I anticipate I will not put it to use. When I can no longer transfer to the front seat, we plan to buy a different vehicle. I don't want to ride around without being able to see out the windows.
The van had about 82,000 miles on it when we bought it used. It needed a bit of work to bring it up to standards I was comfortable with. It has been reliable and has suited our current needs very well.
The EZ Lock wheelchair locking system required us to spend about $700 additional dollars to have it configured for my wheelchair (moved the EZ Lock base and installed the EZ Lock bracket on the wheelchair). We could have saved considerable money if we had purchased a used EZ Lock bracket for my wheelchair, but we did not know that was an option.
Be aware that negotiating the lift (rather than a ramp) while sitting in a wheelchair may be a bit intimidating for some. Others may find it totally impractical. It works very well for me.
The bottom line for me was that getting the van was transformational. I can now get out of the house and go wherever I want to whenever I want to. It has been a big part of restoring my freedom.
Good luck finding a vehicle that suits your needs.
Steve