I've run an older Smithy for about 10 years. Can't imagine getting along without it. I have purchased a lot of tooling here and there as needed to meet some particular job, and that will run up your total investment. However, the same is true regardless of what lathe or mill you buy and lot of the tooling will work with any future machine.
The Smithy is really useful for turning out small parts quickly, things like axle spacers, tailwheel bushings, wheel pant hangers, and bearing stops. I also use it a lot to simply square the end on a tube, or the end of a mandrel. These kinds of parts don't require extreme accuracy. When I need .0005 accuracy (like a perfect shaft fit in a bearing) I throw a rag across the bed to protect it and switch to abrasives for the last fuzz of material removal. A right angle die grinder running a 3M deburring pad can be used like a hand-held tool post grinder. Internal blind bores at the .0005 level can be done, but you spend a lot of time and cross your fingers for luck.
The mill function is useless for large surfaces. Not enough bed travel in any small lathe/mill. Also, all the lathe/mill combinations require that the part be elevated a good bit above the bed, which introduces flexibility.
The advertised swing is misleading. The practical limits are about 6"D for aluminum and maybe 3.5"D for steel. You can't slow the spindle enough to get a reasonable cutting speed. Smithy does sell a planetary speed reducer pulley, but it is junk. I killed two of them and started taking larger diameter jobs to a friend's house to cut on his big lathe.
Which brings us to a side advantage. Turn out nice parts on your Smithy and pretty soon the other lathe and mill owners in your area offer to let you use their larger machines when needed. A Smithy is a great training tool because you must think your way around how to get the job done, and as it is belt driven, gross cutting errors tend to result in belt slip, not broken parts.
None of the above is to say you can't turn out significant work. I've used mine to build a complete propeller speed reduction unit, an experimental viscous torsional vibration damper, wheel hubs for WW1 replica wire wheels, you name it.
I think buying one depends on what you see in your airplane future. I TC a lot of projects and assist with a lot of repairs, plus my airplane world includes custom tube and fabric. Right offhand I can't think of very many parts I've made for RV's.
Dan Horton