If this is the only project you'll face in several years, just let the bike store guy do it. You'll spend some money buying tools. The payback comes on the second bike. I do my own work, but that's just because I want to. Not because it makes sense financially. I have an old MTB converted to SS which I rode all last winter. I run 35x16 gears and 26 x 1.5 inch tires. The magic off road gear ratio is 2:1. I think that is a good starting point for winter riding.

What I did for the BB is put my cranks on the old BB. Measure the chainline error. Measure the width of the old BB. Then figure the difference required to correct the error.

I like the 110 BCD cranks for this application. Lots of BMX SS chainwheels to choose from. My 36t is a stainless steel Surly.

You've got horizontal dropouts on your frame, right?