Did you have a look at the Sherpa brochure? They have a pretty simple chart near the end. 5'7" should be around 170cm. Thorn suggests frames between 530 and 565. If you have an existing bike that fits you well, you should have an idea what size you need: 530 means 530mm or 53cm. S, L and XL only indicate top tube length. First select the seat tube length you need, and then look which top tube sizes are available. Again, if you have a bike you like, simply measure the top tube length and select something similar. You could also e-mail Thorn. They'll be able to help you.
With regards to building up your own frame: don't do it to save money, unless you already have most parts. You probably can save money, but it'll cost a lot of time, and you should know what you're doing. I built up a Sherpa, but I did it for fun and because I knew that I wanted a very different spec. I pretty much ended up spending about as much as I would have paid for a complete bike. I had a lot of fun, though, and I like tinkering with my bikes.