Alan
Let me preface this by saying I have never used a 3-in-1 (hence the reason I started this thread).
1. Is there a learning downside to the 3-in-1s? No. When I was in graduate school (when CNC was first finding its way into schools), we used a complete micro CNC "machine shop" on which we learned the fundamentals. The machines were too small to really create anything useful, but they demonstrated the theory of how everything worked. Now, we weren't learning how to use a mill or lathe per se, rather learning how to program CNC equipment for tool changes, feed rates, patterns, etc. But, I think the analogy to a 3-in-1 is apt. Using one of those combo machines, you should be able to learn how to mill and turn quite effectively. The two functions are independant, just completed using the same chassis, so I think the skill transfer will be there should you ever find yourself on a "real" machine tool.
2. Can going slower with less of a bite overcome rigidity/power deficiencies? Yes. Cutter speed, cutting depth, feed rate, and machine rigidity all interact. If the latter is less than desirable, the others need to be adjusted. Think about cutting a big block of hardwood on a cheap-ass table saw. If the saw is not up to the task, you take a smaller bite and go slower, you just have to make multiple swipes.
3. Tools and other ancillaries. I don't know off the top of my head what HF, Grizzly, ENCO, etc., include with their packages. I don't think HF includes anything (Randy or Ron may be able to comment here), but I think ENCO includes stuff and Grizzly may, too. Smithy definitely does. So, my point here is, you really have to look at what you are getting for a stated price. A $1500 machine tool may not be any cheaper than a $2500 machine tool if it doesn't come with any ancillaries and the other does. As far as what tools you need, the ones that come with Smithy will get you going, so other than different-size end mills (if even those), you shouldn't need to invest any more right away. And, you can add other ancillaries bit by bit (no pun) as you go. I would, however, invest in a live center when you get to the point of doing a lot of turning with the lathe. Honestly, I have not priced the ancillaries separately, so I can't really comment on how much is "enough" when purchasing them by themselves, but "as much as the mill" seems too high.
(I'm not trying to sway anybody to Smithy, it just happens to be the brand I have spent the most time looking at, so am most familiar with.)
As far as the mini lathe (or mini mill), I looked at those a couple of years ago, but quickly dismissed them. The price is very attractive, but they are just too damn small. Your hobby would have to be something like model railroading, I think, to really be able to get any real use out of one of those. I think for what we are discussing, it would be a mistake to get one of those.
The things I look for when comparing machines are:
lathe: center to center distance, throw (diameter of workpiece), power feed (needed for cutting threads), and thread range; one of the things drawing me to the top-tier Smithy is its infinitely adjustable speeds...very important to the former machinist in me
mill: X, Y, and Z travel (especially Z...some of the machines I see have only about an 1" of Z travel) and end mill size (chuck capacity diameter); also a consideration is if the head can rotate (not critical, but very useful)
Once you get a lathe, whatever that ends up being, I'd be willing to bet you'll quickly start seeing uses for it. In fact, it wouldn't surprise me if in a few years you find yourself wondering how you ever got along without one.
As stated before, I have not actually used one of the 3-in-1s. And, Alan, I don't know you personally, nor have I seen your needs firsthand. But, after "talking" things over with you on this thread, my gut feeling tells me you're in much the same boat as me, and my gut is telling me a 3-in-1 is the way for me to go. Both Randy and Ron have given their 3-in-1s high marks (even if a tune-up is needed to start), which only reinforces what my gut is telling me for what I need/want. I can't tell you which way to go, nor would I want to do so. So, take all of this and evaluate it with your own critical eye.
Let's step back and look at everything realistically. Do a bit a reading about machine tools in the Industrial Revolution and what those guys were able to do with equipment we'd be on the floor laughing about today. As backyard hobbiests, are we dreaming too big just because "bigger" exists?
You mentioned "reasonably well" as a criteria. From what I've read, any of the 3-in-1s can hold at least a +/- .002" tolerance (spend a bit more and get +/-.001"). A tolerance of +/- .005" is (or used to be, pre-CNC) a standard spec for machine work. Anything tighter than that was usually considered precision work. Point being, even if the manufacturers are stretching a bit and their tolerances are closer to +/-.003" or even +/-.004", you're still under what used to be considered a standard tolerance.
By the way, you mentioned creating brackets with a nice curve. That's going to take some skill on a non-CNC machine. Not impossible, but definitely something that will take some practice.
Well, Alan, even if neither of us ends up with any machine tool, I've enjoyed the discussion.
Eric