Which fan for Taz 2?

I have a Taz 2 and want to add this extruder fan https://www.lulzbot.com/store/parts/taz-extruder-fan-add-kit

Which voltage should I choose?

Thats going to depend. All of the Taz 1’s were 12 volt. Most of the Taz 2’s were as well, but there are afew older ones that were 24 volt, and all Taz 3’s were 24 volt. The control board will take 24v, and there is an upgrade proceedure here https://ohai.lulzbot.com/project/taz_12v_to_24v/maintenance-repairs/

you can tell for certain by looking at the model information on the power supply. if you can, take a picture of the label and post it here. in all likelyhood chances are it’s the 12v model. if it is 24v and you attach a 12v fan, it will burn the fan out but not otherwise harm the board, so there isn’t much risk in trying a 12v fan first. The 24v upgrade is worthwhile if it is 12v, as it decreases bed heating time by half.