there was a JBL D140 (maybe K140) loaded 2x15 that came with the amp originally. Pretty efficient 2x15 cab that would get good results from the 135.
There are newer cab/speaker designs that would also work well too. Barefaced, Bergantino, LDS (made to order), Genz Benz.
Lots depends on what you will use the amp for, and what you are looking for as far as sound and tone.
Key thing for cabs will to look for efficiency (as close to, or over 100dB), and multiple drivers to get the most out of the amps 135w.
Tubes can be visually inspected for obvious issues (cracks, powdery residue inside the tube) but only a qualified tech with a decent tube tester can tell you what shape the tubes are really in. Tubes can, and do. last a very long time if they are taken care of, so just because a 70's amp has its original tubes, does not mean they still don't have plenty of life in them.
Another thing to consider is that the amp may need a tech to check and replace some of the capacitors (caps) as they do have a finite operational life.
Turn it on, plug your bass in and see how it sounds. Bad noises (hum, buzzing, distortion at low volume, etc) are basic signs that the amp probably needs a good check out by a tech. On the other hand, if it sounds good, and operates for long period of time without changing, its probably ok.
good luck