From what I can gather, it's 3 1/2" centre for ML7 and 3 1/4" for ML10 - both seem to have been available in a number of lengths between centres. Wish I could see them both side by side!
I've just bought a cheap old Atlas lathe. 36in bed, 4in swing, 1/4 horse240v motor. It was small enough to break down and transport in a car and I recently knocked up a stand on castors for it with a swarf tray underneath. It's a bit primitive, doesn't have power feed, but is small enough to move around and suits me for the jobs I need. Unless you really need one and believe you'll be competent on it, I'd go for a cheaper smaller unit.
I agree with the above about safety etc but just get stuck in!
I bought a lathe a couple of years ago ready for my cafe race bike project and I had absolutely no experience of using one.
Just find the one you want get it onto a sturdy bench at the very least and then have a play with all the levers dials etc until you know what they all do. If you can get someone that has a good bit of experience with a lathe buy them a crate or two of their favorite beer and get them round to give it a once over and ask them to set it up.
I didn't have that luxury and it took me a long time to realize that the lathe was playing up causing problems and not my machining skills! (It was the previous owners abuse that caused the problems not really the lathes fault)
Once it's level and tweaked just wack a bit of free cutting mild steel in it and have a go.
This is what I've made.
Some brake disc spacers for the bike. All done on a Chinese machine! (Chester lathe mill combo)
Modified rear wheel hub .