Just picked up on this thread and would like to give my pennorth. Would echo on thoughts of going 'as big as you can', but feel it only fair to raise the issue of weight. If you want to be able to move the machine around on your own, then a Unimat 3/Sieg C0 lathe or a Sieg C1 is about the limit. The 'mini lathes' that everyone sells are a 2 man (person?) lift, unless you want to break it down to its components to move it each time (and that also means single-handedly taking it out of the box...).
It is surprising what you can do with a smaller machine, especially as regards small details, bushings and shafts. The Sieg C1 actually comes with the same 3 jaw chuck as most 'normal' mini lathes in standard trim, and in my opinion has a far more rigid tool post than its bigger brother. Build quality of the smaller C1 is also better in my opinion.
If you want to turn 1/6 scale turret slew rings and barrels, then you really are going to be looking at something around the size of a 'real' lathe, with about 230mm swing - that's the size of what you can rotate above the lathe bed - and able to take anything up to 900 or so between centres. If you are wanting to deep drill barrels of this length from solid bar stock - that's about 15 - 17 feet - one is into gun drills and all sorts of clever boring tackle (which as a newbie I'm only qualified to mention in hushed tones, let alone even thinking about having a go..).
Chinese-made machine tools are no longer the bargain that they once were, and it may be that you want to look at older British stuff as well. Go to
http://www.lathes.co.uk for more info on machine tools.
There are still a few ex-schools Boxford lathes that come up on ebay (spit) from time to time, though you will need to budget for another 200 squid to convert most of these to single phase as they are normally 3 phase electrics. Been there, done that, got one in the garage. The Myford ML7 and its ilk (Super 7, ML10) are in-betweenies because (a) they are overrated (again in my opinion) and (b) usually overpriced compared to what you get from a 'bigger' lathe. A good selection of spares is still available for the older - 1950-1980s -Boxfords, and basically the same design is still made by South Bend in the USA (but you really don't want to ask the price...) There is also some spares support for the Smart and Brown machines, if memory serves me right.
Last comment here would be come back to portability. If you put the machine on a set of lockable castors, you can easily move it around the garage or workshop. For when you drop that last M10 nut down the back.... Castors on my machine are mounted to angle irons, which are then offered up and bolted through the cabinet end plating.
