@Bone Head , knowing what you do and your tolerance for bs I have a specific suggestion.
Aside from budget restrictions I think you are looking for a Quick Change Gear Box.
There are plenty of threading jobs in your life. I'm sure they can be done with change gears but...
At least a 12" so you can use the centers to hang wheels (Crankshaft for the rest of you) with room to spare.
It's never so true that you can always make small parts on a bigger lathe but you can't make larger parts on a small lathe.
In any case skip the 6" no matter how "cute" . 9" with gearbox are scarce. 10's with and without gearbox. With 12" you usually have at least the option of a gearbox and power cross feed.
There's more but I think you get the idea. Set a foot print, function, and dollar budget. Be patient but ready to move instantly when the deal arrives.
The one thing we all will warn you about. Once the lathe arrives and it's presence has caused a bit of musical chairs in the shop you're only getting started. Just the tooling I have for the 6" cost far more than the lathe. Some here have kept track and estimate that the tooling may be twice what the cost of the lathe was. There is one small bit of relief here. For those of us that have mills or access to them have made a lot of our tooling. We're not going to be building chucks and collets (well, emergency collets but that's in the advanced course) but tool holders and other accessories can be made on the lathe or mill. A quick note on milling attachments for the lathe. It's a bit of a bodge as the brits say . More like a kludge or a half way measure that just doesn't get it done. I mean, they work, but in such a limited sense that I wonder why I have one. Oh, right , I didn't have a mill at the time and needed some key way slots ....