Let me throw my 2 cents worth in, might be helpful might not......
I've been driving manual commies for awhile now, even converted a few for mates & I've been running the billet products short shifter for a couple of years as well.
Start with the easy one, what ever box you decide on put a shifter on it!!! It is by far one of the best things I've ever done & now have numerious mates using them based just on the performace see in my cars.
I belive the T5 is a better box, but that's because I've only used them in my cars. As for ease of use, the T5 & gertrag are about the same, but the T5 is a bit easier to remove by yourself as you can split the box from the bell housing while it's still in the car. You can use a solid fly wheel with either box.
If using a T5 I would deffently recommend using the V8 version over the V6 one as it is a bit stronger, the case is the same & will bolt up to either engine with the right bell housing. Again they are the same from VN-VS1 & I would highly recommend using a brand new fly wheel when doing the conversion, along with a brand new clutch set up (Might sound simple, but Ive seen people skip this step & pay for it later)
Guessing by the buy-in price I'm gonna assume that this is for a VS??? if this is the case, you will need the following items to carry out the conversion:
Gearbox & bell housing
Manual tail shaft
Gearbox mount
fly wheel
clutch kit
pedel box
cable/master & slave cylinder (depending on which way you go)
Manual computer (as the VS uses the computer to control full operation of the auto & will try and find the signal if not changed, there might be a patch for this but I'm not 100%)
The inhibitor switch will need to be by-passed
All other assc. mounting hardware.
I think I've remembered everything & I hope this will help you.
Keep us posted on your progress.