I'm not sure what software soup uses, but I can ask. I use an old version of GIMP from before they got rid of the alpha later feature, It's like version 2.8, and it is free. Though I know most translation groups use Photoshop.
You don't necessarily need a drawing tablet for typesetting, but I can see how it would make cleaning faster, and you'd probably need it if you're doing redrawing.

How long does it usually take you and Soup to clean and typeset the chapters? Also what app/program are you using. I'm asking because I used to do proofering for another group a long time ago (had to stop because of uni). But I never asked the typesetters what programs they used or how they saved the files etc etc. At that time I didn't have a drawing tablet so I never even thought to ask about the process because there was no way for me to help (most of them used digital art drawing tablets to make the process faster).