well I don't think gutting it and turning it into a retrofit will do much good for the things you want to do. the only thing you really can save when gutting it would be the bulbs anyway.
if your gonna get a diff. fixture or add a fixture check out the nova extreme pro T5 fixtures they have individual reflectors for each bulb.
if you wanna do a retofit unit. all you would need would be 4 bulbs, 4 SLR reflecors, 4 sets of waterproof endcaps and standoffs and an icecap 660 ballast. or just check out reefgeek.com for complete retro units.