It is rather unlikely that you really want a densitometer. Zone system beginners often believe they do, as they are referred to in books that are older than the scanner era. But that is merely due to nothing better being available back then. As a matter of fact, a densitometer essentially is a manual scanner with one very large solitary pixel. If you want to test your processing and exposure to house standards, any scanner will do better (working on the full film size in one go, with spot sizes small enough to work on actual subjects rather than test strips only).
If you want to test to absolute standards, a recently calibrated densitometer is the way to go - but for that you would depend on test materials that are unobtainable these days (there would be a tiny niche for pro densitometer use if you want it for colour, but are no factory pre-exposed black and white test strips any more, except for X-ray film, which has completely different parameters).
Pro densitometers are dirt cheap, having once been a common industry appliance, and unless you run into something for very opaque materials (they were also used to test fabrics or paper), all will do. Get one for which there are comprehensible calibration instructions online, and which uses a lamp that is still obtainable!
