So yeah, it only took a few weeks for me to break down and buy a real milling machine. Like you didn't see that coming.
Actually I hadn't intended to get one right away, but I was unexpectedly lucky on eBay and got a very nice 12" Sherline mill to go with my 3.5×17" Sherline lathe. Judging from the few parts I've made with it so far, I can definitely see that a dedicated machine is a lot nicer and more capable than a lathe with a milling conversion attached.
The mill seems to have hardly been used at all, but since it's at least fifteen years old and doesn't seem to have gotten much TLC during its life, I decided to strip it down to its component parts so I could clean and lubricate everything. I also replaced the AC cord and switch, and added a power indicator safety lamp as I did with the lathe. After putting everything together, the mill worked a lot smoother and with less backlash, so it was worth the effort.
Then while I was at it, I tore down the lathe to individual pieces too, and gave it the same tune-up treatment. Then for some reason I decided to really get crazy and add DRO scales to it too. That was more challenging than I initially thought, and took a couple weeks to really get sorted out, but it did give me a chance to use the new mill to make real parts. And now that the lathe is back together, it too works better and has digital readouts to boot. (I may add DRO to the mill as well, but if I do I'll almost certainly take the easy way out and buy a kit!)
Hopefully this will all come in handy for the airplane project. Don't worry, I haven't totally forgotten that I have one of those too.