Condition and size of the frame are important. Tons of rust is bad (especially on a thin walled light weight bike). If they aren't disassembled, you can test ride them. If he just has bare frames hanging from the ceiling, you'll need some advice from the guy selling them about which frames will fit you.

Does the guy who converts old bikes to SS put on new wheels? Probably not usually. Check out the wheels on the bikes before choosing one. Alot of older 10-speed bikes had steel wheels which are heavy and don't work so well with modern brake pads. A magnet will tell you if the wheels are aluminum or steel. Note the tire size too. Most will be 27 inch wheels. If you can find one that has 700c wheels, that's better. You can still find 27 inch tires, but you won't have as many choices as you would with 700c tires. If you can find a bike that fits you which has 700c aluminum wheels, then put it at the top of your list.

You mentioned riding on the shoulder and using wide tires. If it's a gravel shoulder, maybe think about a 1980's steel rigid MTB. Some of them have horizontal dropouts and make good SS commuters. I have one of these. The frames are maybe not quite as cheap as the ubiquitous 10-speed sport tourer. I paid $100 for a 1980's Specialized Hardrock MTB. My recently acquired 1980's Raleigh 12-speed with 27 inch aluminum rims was $15. Both had a small amount of rust. No problem with these gas pipe frames. Sanding the rust just makes them lighter.

Edit: 17 miles is a pretty long commute. If the route is mostly pavement, you would be better off with a road bike (aka 10 or 12 speed sport touring bike with 27 inch or 700c tires). They are more efficient than MTB's on pavement due to rider position, tire width, and tire circumference. I ride on the gravel shoulder for a long stretch of my 5 mile commute. MTB tires are good in that situation (especially if the ground is wet).