Someone else here can probably better elaborate with details, but if you are going with the 5-speed manual conversion the FHN transmission is the best gear ratio to match up with our BHW/B5.5 TDI motors. FHN was sold in Europe and you can source it from Frans and other places, and from time to time they pop up over here from people who are abandoning project plans for the conversion but already have the parts.
FHN transmissions are harder to come by because they are higher demand for these conversions. GGB and DUK transmissions are next best options, but there are compromises even with those and those aren't always easy to source.
I held out several weeks for Frans to find a FHN and went with it even though it had higher miles on it than some other options. Some of the other 5-speed manual transmission options can run 5-7 mph lower speed at 2000 RPM (in 5th) compared to FHN. And when rowing through the gears, some of the middle gears are closer together or (alternatively) have more of a drop off between shifts. I don't have much to compare it to, but other folks here who have tried several different 5-speed manual transmission options can better weigh in. One of those wise men suggested to me to hold out for FHN and that it's worth the wait and hold-out. I couldn't be happier that I did. And paired with the DUK DMF clutch kit, it's a great option and should have left the factory that way.
Also, whatever tune you decide to go with, get a Stage 2. Don't be shy to save $50 or $100 on a $$$ project. Stage 2 is much more funner (or most funnest?) to drive....