I dunno about EBC Reds... those will fade faster than the EBC Greens! The reds are their most streetable pads, I was going to put them on Lisa's Saab until I found a better price on the OEM pads. Did you mean EBC Yellows?
I actually think the plan for R4's + SS lines (properly installed to prevent any rubbing) + good fluid is a great way to combat brake fade, and still have a streetable setup for inter-stage driving.
I dunno how much help lexan will be for guarding the tank... I would think that anything hitting hard enough to dent the tank will still dent the tank through the lexan? But sheet metal probably isn't going to be much stronger (though it won't crack), and will weigh more... so I guess lexan is better than nothing, especially if you can do it for cheap.
I wonder how hard it would be to switch to a reflashable OBD-II ECU with you existing engine harness? Can you open source reflash a newer Impreza L/RS ECU? That might be a cheaper way to get engine management. Then again, if you just pick up a low-ish mileage long block from the dismantlers as a primary then retire your flakey motor as a spare, you don't really need EM, since the motor will still be stock.
And, there are no stock LSD's for FWD Imprezas... in fact, after further thought, how can you put an LSD in a FWD car if there's no separate fluid for the front diff? Are there LSD's that will be happy in normal gear oil? I know the STi 6MT has a separate case for the front diff because it can't share the other gear oil. I gotta figure the LSD setup is going to have to be something specific for rally, and expensive.