I would suggest getting either a urethane or abs plastic front bumper as that will take most of the scrapes or bumps if you ever encounter any. The sideskirts, and rear bumper can be fiberglass as they usually dont make contact with anything. MAYBE the side skirts might scrape over certain speedbumps, but if you're carefull you shouldn't have any problems.
As for spoiliers, i would suggest that if you get fiberglass to reinforce the underside of it, or what I like to do is drill through the spoiler lengthwise and install a metal rod inside the spoiler to give it strength, and then i fiberglass over the holes and paint it.
Reason i do this is because with fiberglass, especially with larger spoilers like the reiger infinity ii spoiler, they begin to sag in the middle over time, especially after hot days, and the sagging can sometimes be permenant or cause cracking =/
'94 Supercharged E36 325is (150,000 original km's)
-Vortech V9 G-Trim @ ~10psi w/ BOV
-255LPH Walbro fuel pump
-TEIN SS Coilovers w/ Pillowball mounts
-Keskin KT1 17x8.5 & 17x10
-Remus full catback
-Authentic Rieger Infinity I body kit
-SMT6 EMS (Untuned)