I haven't posted much on the site but have been following the topics since I found out about this project. Some may remember me from the TW era as I was a designer for that series along with a community site owner for many years. I was also a beta tester for the TW series and a community contact with EA directly.
My input on this subject is the following:
1. Two small groups would be best for getting the feedback needed. One group that wants to test all aspects of the game and one that is more geared towards the actual game play. This will allow for better input towards all areas while not having to many testers out there that are not providing feedback.
2. Paid participation may give the best team to assist in the testing as they have a vested value in the game and for the overall outcome. Steam may be an option, but as others have stated, you may get people wishing to just get a discount without providing feedback. It may also cause to large of a group having access to the Suite of software that isn't assisting in making the project better. Maybe you could consider setting up a system that allows those that are chosen for testing to provide a donation (amount to be decided based on testing areas) via the forum which would promote them to the beta group they were chosen for. This would then give them access to a special area on the forums for communicating with the development team.
3. From past experience in beta testing, I would ensure you give the group(s) direction in what areas need testing each round to ensure feedback is provided in a timely manner. Some may wish to only test the game itself, others will want to test all aspects including course building and importing of terrain, etc. Ensure the criteria are broken down for each team of testers as outlined above.
This all being said, I would be willing to assist in any testing going forward no matter what direction your team decides to choose. I am also familiar with the forum software currently being used by PP and could assist in anyway with setting up the areas I've outlined above.