This is quite a complicated question and actually it kind of applies to all of the German pianos except for Steinway (I'm counting Hamburg Steinway as German for this). Steinway's plate and rim design, while not exactly unchanged over the last 100 years, has remained constant enough that Steinway Hamburg (and presumably New York) can rebuild a 100 year old Steinway B and turn it out like a current model. Bechstein and Blüthner can't actually do this because the designs have changed. Grotrian as well - their designs have changed dramatically. Blüthners have actually changed less than one might think, the DNA is still the same, although things have been changed to make the piano more projecting than it was 100 years ago.
Therein lies a central issue. The Bechstein of 100 years ago was built to sound clear and somewhat ringing, it really was a singing tone with a lot of colour in it, and it seemed to be that smaller venues, theatres, and music teachers rooms were all attracted to that sound. What it wasn't, really, was incredibly powerful. It was always a kind of delicate sound in comparison to Steinway. When we talk about Liszt having a Bechstein that he found far more powerful than the other pianos of his day, don't think Steinway, think Erard or Streicher, or Graff.
I *think* that the first of the post-war Bechsteins were incredibly similar in design to the pre-war Bechsteins and that these designs were in use until at least 1990. At some point in the 1990s or maybe later, Bechstein changed the design and it now looks more like a Hamburg Steinway. In some ways the actual tone still has a lot of the signature Bechstein clarity and ring, but it has a lot more power. The rims are much heavier, the plates are heavier, everything is bigger. I think C. Bechstein is a wonderful piano, both new and old, but the two instruments are built to a different ethic.
What would I look for? Well, I think they all have their good points. A turn of the century Bechstein would probably need to be rebuilt if it hadn't been already, and it seems about half of them suffer from cracked plates, but that can be very easily repaired in the rebuilding process. Most of the time the cracks in the plate are of little to no concern, and have often been there from quite early on in the life of the piano, but if I was having the piano rebuilt I'd want it fixed.
If money was no object and I was definitely set on buying a Bechstein grand for my music room, I'd buy a brand new concert A because I believe that the pianos of today are built with better materials and greater precision than the pianos of 100, 70, 50, even 30 years ago. In my opinion which may be subjective, the Bechsteins of today are objectively better. I also believe that the Blüthner of today is objectively better than the Blüthner of 100 years ago although the Blüthner of 100 years ago was exquisite as well. The same with Steinway, I think a new model B would be a far more musically rewarding instrument than a restored one because of the precision engineering used in the plates and other parts, particularly if I was buying a New York Steinway, I'd want the latest model I could find and afford and have it prepared by the best tech I could afford.
That's the other issue, when you've got your piano you need to decide who you're going to entrust the maintenance to, and in a way that's more important than even which brand of piano you buy. In the hands of the right technician even the humble Yamaha C3 can have a glorious and blooming tone and in the hands of the wrong technician a Bösendorfer 200 can play like a pig (and I've experienced both of these specific situations personally!)
