A belated attempt to answer square_wave's question:
There are definitely sonic goals in rebuilding vintage idlers. They are known to have lots of drive that propels the music ever forward. That's because they operate by brute force. Their (vintage idler TTs) motors are typically AC synchronous motors, rated at anything from 15W to 20W. They have lots of torque, and that is a blessing as well as a bit of problem. Vintage TTs were typically housed in box type plinths. The box plinths became resonating chambers for the vibration induced by the powerful motors on the top plate which eventually gets conveyed to the plinth. So when these TTs were in production, they were not rated too highly from measurements perspective. Jean Nantais, the gentleman from Canada who brought about the renaissance of Lencos recognised that Lencos needed mass loading to drain away the vibrations from the motor. Hence the heavy and massive plinths so popular with Lencos.
Sonically, they produce a lively and dynamic sound, with lots of sense of drive. Bass can be thunderous with the right electronics and speakers. The one area where they may not measure up to the better modern belt drives is in digging out finer nuances. Their sound signature is very akin to other idler drives like the Garrard 301 or 401 but not similar to a hybrid like the Thorens TD124 (which has a more subdued signature).
PS: a good belt drive TT like the legendary Linn Sondek or even lesser TTs like the Thorens TD 160, 150, 125, etc can also have a very dynamic drive, provided one tunes the suspension well.
PS2: one can lavish good arm and cartridge on as Lenco rebuild project. For example, the Rega RB330 arm that mpw has bought is by no means cheap. The loaner Origin Live Silver Mark III arm in use right now costs much more than the RB330. So "outlay" can be modest to fairly high.