I would buy a modern ML barrel, from a reputable manufacturer if you decide to go the total DIY route. From what I understand, most modern shotgun barrels don't have a thick enough breach to support the threads from a plug.
I recently finished building a Pedersoli SxS 12 guage kit, it was the first gun I've ever put together. I don't know your skill level, but I'll say this. I'm a pretty crafty guy, I do all of the work on my own home, cars, and I love building mechanical and electrical things in my spare time. My friends and family tell me I'm pretty good at it. If I had tried to build my first gun from mixed parts, it would have never been finished, undoubtedly. I would have never been able to inlet the stock properly without having seen and shaped one that was properly inletted already.
I also had enough trouble trying to fit together parts that I knew were supposed to fit together, trying to match up thread pitches and drill perfectly centered holes in parts from different manufacturers would have just been too much for a first project.
Anyway, what I'm getting at is suggesting that you buy a kit to start with. You know you'll have all of the parts, they'll fit together, and most of them come with some kind of warranty if you end up missing a few parts, or they're broken. You can always sell the kit when you're finished, and put that money towards your custom project as well.
I think now that I have built one from a kit, and understand the basic functionings of a lock, trigger mechanism, and how it all fits together, I could probably build one from parts.