As has been said above, I would be more concerned about synergy with your current speakers than buying a vintage tube amp. That, and the quality of the workmanship with the mod.
Vintage tube amps are really basic devices. Capacitors, resistors, transformers and chokes. There should be no unobtanium parts inside.
Service manuals and schematics are easy to find.
If you live in or near a major center, I would consider looking into what service facilitates exist near by should it need any TLC.