Hi Adam, and welcome to A2OC.
With regards to the 6spd is there only one option available?
There are basically two methods of upgrading to 6 gears. One method is to simply swap the whole gearbox for a 6-speed 'box with appropriate ratios. By far the most common 6-speed gearbox used is the MYP gearbox from a 2012 Passat. The other method adds a 6th gear to your existing 5-speed gearbox, which is detailed in this thread:
https://www.a2oc.net/community/index.php?threads/gearbox-recondition-and-6-speed-conversion.21523/
There are pros and cons to both methods.
The MYP gearbox is designed for cars that use a dual-mass flywheel and that use the ABS system as the instrument cluster's source of speed information. In other words, the bell-housing is deep, to accommodate a dual-mass flywheel, and there's no speed probe/sensor inside the gearbox itself. The only A2 that fits this criteria is the TDI90. All TDI75s use a single-mass flywheel, and earlier TDI75s need a speed probe/sensor inside the gearbox in order to get road speed information. As such, in order to make the MYP gearbox compatible with the TDI75, some swapping of gearbox components is required. This additional work can add to the costs of the upgrade significantly.
The MYP gearbox uses a shorter shift gate, which translates to a shorter throw at the gearknob itself. Some don't like this, as it can make the gear change even more notchy than it is already. Also, the gears are arranged inside the gearbox in a conventional order (1, 2, 3, 4, 5, 6), meaning that the tallest gear is right at the end of the shafts, as far from the clutch as possible, which is less than ideal as it increases the strain on the shaft bearings.
Adding a 6th gear to your existing gearbox is, I believe, the more elegant solution. However, you need to have confidence in the good condition of your existing gearbox, and it's a costlier option. The gears are arranged inside the gearbox as follows: 1, 2, 3, 4, 6, 5. The result is that the tallest gear sits evenly between two of the three shaft bearings. Also, the 6th gear is marginally longer, meaning lower revs once motorway cruising.
Once you've found the right car, I'm sure you'll end up talking to @timmus
Thank you, Proghound, for the recommendation. Once the Covid-19 crisis is over, and Boris has let us all out to play again, my A2 upgrade service will be back in full swing. Let me know your wish-list of optional extras, and I can make it happen.
Kind regards,
Tom