as a buffer, you'll just be assisting with dmg, and healing from time to time. hopefully your main will fight in an area where he can rely 90% of the time on healing herbs, else your wc is gonna be hurting bad for mp after the round.
as it is, at level 40, you will not be pvping, nor doing any real type of magic damage (for the most part). go for the plated leather light armor set and a shield, dark screamer/focus (or beserker blade/focus). the mp regen on the armor will help for buffing and it keeps the mp pool higher than heavy, while allowing use of soul cry a bit more than when in heavy. you can only fight in places where herbs drop, as far as i have noticed on my lvl 55 wc. vr1 just doesnt do enough, nor do the mediocre buffs wc's get in the lower lvls.
if you run every single buff in the game, you might have issues with mp in light armor till your mid 60's, so that is the only reason to use robes imho, or unless you're in pvp. acu mage wep is nice to expedite buff time but not too useful as an assisting dmg weapon.