If you go PRO you will only be sacrificing your PRO cards as you will still be able to run the standard missions with your non-PRO cards.
Regarding the levelling and the success of the band. We are still working on the details but it will probably be better to have more bands and level them up than have just one band. The idea is each band can only have one single in the chart at a time and the higher the level of the band the longer the single will stay in the charts.