if you are sure you need a walking foot machine, you may want to consider a vintage Davis needle feed machine. If you get a later model they take current production size needles, so you only have to worry about getting vintage shuttles and bobbins for them. They aren't that hard to come by.
I've sewn on several different needle feed industrials, the two factories I've worked at did not have any machines with walking feet. One sewed heavy coats and the other draperies.
I've been sewing for ages, and have yet to actually use a walking foot for anything, and I've done a huge range of sewing, from custom lingerie and wedding dresses, to auto upholestery and large canvas tent repair (had to set up on the lawn for that one).
Singer does not have the reputation it once had, you may to better to look at Brother, Juki or some of the other companies that are really known for quality industrials now days. Though if you get a vintage Singer industrial, they are very good.