I just set up a new 28g Euro tank with a 10g sump below, i added a 18'' Zoo Med coral sun light. Would this light alone be enough for a future reef tank or will i need to get more?
How long is this tank? You can probably get away with 1 250 watt metal halide. I would go with a a 14K bulb if you don't want to supplement the halide with actnics. I like a 10K bulb backed up by two actnics.
Lighting is a major component when setting up a tank and you do not want to skimp.
What sort of reef tank are you talking about. Power compact or VHO lights are generally enough for most soft corals and a few of the less demanding hard corals. If you're talking about a reef tank like you see in some of the pictures around here, metal halides or quality T-5 HO lighting is definitely the way to go.