The P-ROC board will provide you with a clean slate and if you go that route you will have the ability to change everything including the sounds and light shows and, as such, you would not need the pinsound or led ocd boards. Again, to be clear, you do not need those other two if you have a p-roc!
The p-roc connects directly to any WPC machine (pre-fliptronics through wpc-95) replacing the MPU and interfacing directly to the switch matrix, dmd, and existing driver boards in that architecture.
Once you have replaced your MPU with the proc, you connect the proc to any sort of controller pc via usb (where that can be as small as a beaglebone black, raspberry pi, or small system on a chip linux or windows system --or a full fledged machine running windows, OS X or linux). Sound will come from the PC. You may interface to the existing dmd via the proc or you can replace your dmd with an lcd display and go with a full display update as well.
There are several software platforms for game development on the p-roc: pyprocgame, PyprocgameHD+SkeletonGame, and the aforementioned MPF.
MPF remains in its early stages but there are several complete rethemed or "continued" games built on PyProcGame including cactus canyon continued, BoP2, F14 second sortie, and Earthshaker aftershock. Buffy, Pinbot 2.0 and some other unannounced rethemed/continued's all run on PyProcGameHD. There are projects on MPF, but I'm not familiar with them.
Oh and if you're running a proc you can also run pinmame to emulate the original ROMs and play the original game on your machine while you're developing your game, so it won't require a board swap while you're programming your own game.
If you want to learn about proc programming, head over to the forums at pinballcontrollers.com; that's likely your best bet. Check out all the projects there:
http://www.pinballcontrollers.com/forum/index.php?board=17.0
I have a site I maintain for PyprocgameHD+SkeletonGame, which is http://pinballprogramming.com
