The 111W is the original industrial compound feed walking foot sewing machine that every other sewing machine maker copied. It would be hands down the better choice to do car upholstery. I would think the 111W153 would be 3 or 4 times more than the 500A. You are talking about the difference between a Ferrari and a Yugo.
The 500A is a home sewing machine with needle feed only, and it does not have a walking foot. "Walking foot" is a generic term for alternating presser feet. This means there are three shafts, one that holds the needle, and the other two each hold one part of a two piece foot. When one part of the foot is down, the other part is up and vice versa, hence the term alternating presser feet. You can't add an attachment to any sewing machine to get a walking foot, it either has it built in or it doesn't.
No one lives forever, the trick is creating something that will.