My two pennyworth.
I had a South Bend, like the Boxford, and access to my dad's ML7 when I bought another lathe.
Three things drove me to drink with the SB and M7.
1 – That the chucks were held on a screw thread, meaning that grief had to be imparted to get them off. A lathe with a taper fitting, my Harrison has an L2 taper, or the newer Camlock type are really worth the cost.
2 – That the SB etc had a spindle bore that seemed to allow only a pencil to go through it. The Harrison has 1.3/8" and that I would say is a good size.
3 – That some form of quick change gearbox is essential. My Harrison has a full Norton box, but the alternative of just three speeds or pitches is worth its weight in gold. I do almost no threading other than at 48tpi, so changing gears wouldn't be too much of a pain. But feed rates are a completely different thing, and being able to quickly change is essential otherwise you just don't bother, and power feed finish is so much better than by hand.
Don't dismiss secondhand, look on lathes.co,uk for a changing selection.
To test a lathe do the following. You will need a micrometer reading to tenths of a thou, or 0.002mm, preferably the fiducial type.
1 – Get a bar, 25-30mm is fine, of free cutting steel in the chuck. Even brass will do.
2 – Get a round nosed tool, needs to cut equaly well to the left and right, with a nose radius of 1mm or so.
3 – With a parting off tool cut two grooves in the end of the bar. One about 10mm from the end and the second another 10mm from the end of the groove just cut. Both grooves need to be about 3mm deep.
4 – Set lathe to run at a suitable speed with a feed rate no coarser than half the radius of the tool.
5 – Position tool in the outer groove with a depth of cut of 1mm at least. Clamp cross slide so it can't move.
6 – Set feed direction to the left.
7 – Start lathe and cut to the left until reaching the inner groove and stop the feed, leave lathe running. So this has surfaced the bar with a 1mm depth of cut.
8 – Quickly move the carriage to the right until the tool is in the outer groove. A spiral cut will appear in the just machined area.
9 – Stop lathe, reverse feed direction to the right, start lathe.
10 – Engage feed so the lathe now cut the bar to the right. When off the end of the bar stop the feed and the lathe.
11 – Measure the diameters of the two sections just machined, one to the left, one to the right.
12 – Weep.
What you have done here is check the saddle for skew as it moves under a small cutting load to the right and left. If the readings agree within 0.01mm then the lathe is good, normally it would be 0.1mm or even worse.
Try this on a new lathe in the showroom and see what excuses are brought forth.
On a second hand lathe, Colchester, Harrison and similar, the bed is hardened and the wear will be in the saddle, not hardened, so it can be scraped out. My Harrison had an error of about 5 thou, but after scraping it is 2-3 tenths.
Good luck!