Parts availability is a concern if the machine is in poor shape or lacks items like steadies, tailstock, change gears (if necessary) or requires replacement of non standard parts like special bearings.
I have a Colchester lathe that although was somewhat common, the parts are tough to find. What saved me was that the machine so well specced and in great condition. The only thing I needed to really find was a fixed steady which I eventually did for the princely sum of $180.
The only reason to upgrade a machine to another in the same general size is if the new machine is more rigid / heavier, is in better general condition (less wear on the ways and leadscrews - not paint), has more functionality (better range of thread pitches, sealed gearbox, clutch and brake etc...), is fully accessorized, or has better features like hardened bed, larger spindle bore, metric and inch dials etc...
I have fixed a few machines now and it's often the case that we get upgradeitis and don't necessarily get better machines, just a different machine.
Paul.