Here is some information on the Minnesota C, which appears to be a Davis machine anyway:
http://ismacs.net/sears/sears.html
Check the price for attachments for the Minnesota on ebay, or any other shopping site that might carry them. Based on treadle prices in my area (only craigslist) I think $200 is too much and I would not drive hours to get it. For either machine make sure all the parts are there, particularly the shuttle, because finding a replacement can be difficult/expensive.
Also, both these machines are vibrating shuttle machines (VSM), which is way different from a round bobbin machine. I don't know if VSMs technically are vertical feed, but I would guess not.