The ZB32/34 series is rare. BSA built both iron and alloy versions of the 32/34 Competition models at the same time for awhile. The barrels and heads are certainly different for the alloy models (and the alloy ones are usually different than GS units with smaller diameter inlets), also on both iron and alloy 32/34's the cranks are usually different and so are the cams. The cases I've seen have more webbing around the drive side bearing and wide flanges, when the B31/33 didn't. Of course the pistons were different as well.
The frames were common to the B31/33's in most circumstances. I have a 1950 ZB34.A and 1953 BB34 iron. I also have a 1953 B33.A, which is extremely rare, but not likely to have great value.
If you want an iron ZB34 and don't wish to pay the rather large sum like that seller is looking for, buy yourself a decent B33 and then pick up a bottom end for 34 on ebay.
This project will never be worth gold star money. The iron barrelled unit is worth pretty well the same as B33, but, you will live the knowledge that you have something special. Also, you can do it up in Trials trim, which is really quite excellent.