For a start, you need to have some idea of the use to which the lathe is going to put, plus space available, and not least, your budget. The neighbours will not thank you for using a noisy machine within a semi detached house, or in a block of flats. Nor will you relish hauling a 200 Kg machine up two flights of stairs!
If in doubt, go a little larger; your horizons WILL expand over time.
If you were into watchmaking, a 21" Dean Smith and grace would be little use, similarly, a watchmakers lathe is no good if your ambition is a 6" scale Foden lorry.
Having said that, it is amazing what has been made on a Myford 7, far in excess of what would be imagined, initially.
Myford used to be almost the standard model maker's lathe, but with anything at least secondhand, beware of the example that is worn or has been abused rather than used. They tend to be pricey, and you can spend quite a bit on new spares or upgrades..
Boxford is another popular make. The later Far Eastern products have improved a lot over the last 15 years or so, in quality, reliability, and specification. For larger, used machines, Colchester and Harrison spring to mind.
A machine ex Industry is almost certain to have three phase motor(s). Equipment to convert single phase supply to three phase, will increase costs, as will changing the motor to single phase, (which some might regard as a retrograde step), if that were to be your chosen solution.
Before the education system lost it's way, and shortly afterwards, machines with a little wear, but some scars would become available from Technical Colleges and Schools, but having rid itself of these nasty dirty practical machines, the supply has virtually dried up.
For secondhand machines, research availability of spares, or accessories. You can get very frustrated not being able to obtain the changewheel that you need to cut an unusual thread, or the steady that you need for a particular job.
There have been many tales of woe about mini lathes with variable speed. Some of this is down to failing to heed the instruction always to start with the speed control set to zero. Hopefully, reports of quality/reliability problems, have resulted in improvements from the manufacturers.
Very often, Far Eastern machines are imported from the same manufacturer, by various companies, differing only in paint scheme or the range of accessories provided as standard.
When I retired in 2003, for roughly the same specification, a new Myford would have cost me four times what I paid for a larger Far Eastern machine, with all the Chucks and Steadies, and hardened bedways, as standard, plus Inverter Variable Speed drive.. No machine is totally ideal, but I have very few complaints about my machine. It does what is required of it, with as great a degree of accuracy as I would expect from a new Myford 7 Series, and is more rigid. (As you might expect from a machine weighing three times as much) It replaced a ML7!
Good luck with your choice!
Howard