Quincy,
I just recently dove into the deep end and am building a very ambitious Stuart triple expansion engine from castings as my first metal working project, having only obtained a lathe and milling machine earlier this year. I started researching what machines to get and how to make various parts similar to those that I would have to make about a year ago. With a little googling, I found and continue to find all sorts of great websites of others who show and tell how they went through building Stuart or other similar steam makes of steam engines. A few builders don't seem to be all that skilled, but most have some pretty amazing skills. There are a few really good online forums including this site that also have a fair number of discussions on building steam or other types of engines from castings or bar stock. I guess what I'm saying is to look around this site, and google "Stuart Steam" "Stuart Engine", etc and be prepared to be overwhelmed.
There are also a number of print publications. There are a few 'how to' books for several of the less advanced Stuart engines. You might also want to search the "Model Engineering" magazine indexes which can e found online, as there are numerous good multi-part articles on building various Stuart and other engines from castings. Back issues of this magazine are readily available from a few UK stores via the Internet. I found a great 8 or so part article on the build of the triple expansion in this magazine, of which I was able to get all but the last part.
Take it slow, do a lot of research, and I think you'll be fine. I've had to go through a very steep learning curve over the last year, with getting machines, learning how to cut metal, and how to make steam engines and have much still to learn, but I'm pretty thrilled with what I've produced so far. There are multiple ways to make just about any part, so dont take any one resource you find too seriously. I found about 10 resources showing how to make a multi-throw crankshaft from a single piece of bar stock, each giving very compelling reasons to support the method they employed. After all that, I came up with my own unique method that worked very well for me.
One of the many things I've learned from googling around is seeing how resourceful people are with building very complex parts to high precision with some of the most basic of machines.
You have done well to start with a Stuart engine, as they appear to be very highly regarded for very good quality castings that are easily to work with, with equally accurate drawings.
Robin