RE: Time to buy a lathe
The only one area that applies to all small lathes which someone has already been mentioned is the spindle size, ie the hole in the middle of the chuck, not the chuck capacity/size itself.
To explain better the chuck is driven by a shaft/spindle which has a hole up it, if you have say a 300mm long bar of 30mm alum, on a big lathe you you just push it into the chuck as far as you want and machine away, on the small lathes you cant do that as the hole in the spindle might only be 25mm and on top of that the hole in the middle of the chuck may reduce this to 20mm. So it limits the lengths of large diameter stock you machine, not the diameter but the length.
It is not really a big problem as what you do is you cut off the piece you want from the long bar, stick it in the chuck and machine away, if you need to machine a long piece then you have to get more inventive with using the tailstock and revolving centres etc.
In order to solve this spindle problem you really have to go to a lot bigger lathe, a friend has one the next size up from me and he still has the same spindle issue so it is something you live with if you want to have a small compact lathe rather than a big machine taking up half the workshop.
For modelling stuff we never need anything that long that it becomes an issue.
Just to add on the machine I have (shown above) it is more than adequate for any modeller, if you come from a tool making background then you may want more but it will do anything most modellers will want.