Hi BT
1) Yes, I think for your planted tank, 4 tubes x 54W (4ft) will do the job fine.
2) Yes, the price of ATI Powermodule went up slightly since day 1.
3) Yes, you can consider other alternatives... No, they cannot compare to ATI Powermodule.
4) I just bought a new fully dimmable ATI Powermodule (4ft x 10 tubes) for my new tank. In order to dim the tubes, you need an external hardware, for example Aquatronica with dimmable module (coming soon).