Nice discussion and this is the South Bend Forum so will answer about South Bends. I have 3 other brands in addition to the South Bends.
I recently finished repairing a sad 14-1/2" x 60" BTC SBL and will say it is a sweet machine however parts are not readily available. I think about 8,000 of these were built versus some 250,000 for the heavy 10. I have used some 13" parts (the taper attachment), some 16" parts (the gear train to drive the quick change gearbox) and the lead screw from a 16"/. I recently bought a box of 13" parts and there was a 14-1/2" steady resst in the box. Good for me.
I have 3 of the 13" lathes and they are just the right size for a home shop. The heavy 10 is preffered for gunsmithing with the 13" being next in line. I have had little trouble finding parts for the 13" lathes and now think I have enough to rebuild at least two of the three and maybe even the third one. South Bend parts are less expensive than any other lathe I have purchased parts for. A couple of parts for my 13" Colchester cost more than I paid for the most expensive used South Bend 13 and I will not even mention my Monarch 10EEs.
I look for lathes from the late 1960's thru the end of South Bend production. I like a 13 with the 1-3/8" spindle hole, a D1-4 or a L00 spindle nose, double tumbler quick change gearbox, a threading dial and a taper attachment. Make sure you get one with a tailstock as they often show up without a tailstock. Tailstocks are hard to find at reasonable prices. I find them for sale for more than I paid for the rest of the lathe.
You can buy an aftermarket toolpost that is much better than the orginal SBL rocker tool post. Try one of the Dorian or Aloris quick change too posts. I am currently using a Royal four position turret tool post until I can find a good buy in a Aloris BXA or the Dorian equivalent.
I prefer a 3 phase motor and I add a VFD to drive it. Variable speed is really nice on these lathes.
13" models are more readily available than 16" models however I can often buy a 16 for less than a 13 as there is not as much demand for them.
I currently have a collection of lathes in this size range:
a colchester 13 x 36 BTC, geared head
a colchester 13 x 24 BTC, geared head
a Hendey GP No 1 x 30, belt drive, which is really 14-1/2 x 30 BTC, 1951 vintage (my most recent project)
two Monarch 10 EEs of 1951 vintage, DC with motor generators
two South Bend 13 x 28 BTC, belt drive
One South Bend 13 x 40 BTC, belt drive
One South Bend 14-1/2 x 60 BTC, belt drive
As one can see, I have enough 13 to 14-1/2 lathes to get some comparison.
I will evently downsize and keep:
South Bend 14-1/2" , one Monarch 10EE and the Hendey No 1 x 30.
Bruce