@Al, I wouldn't be happy with 3 units, I have seen 3 units over a 5ft tank and 2 over a similar sized tank, 2 is too little and 3 a bit too much IMO.

@Matthias, 5 units was my thoughts, the light spread for me would have to be uniformly spread, the corals won't be wall to wall, but would make up a large area in total.

I best get saving up, unless you do interest free credit!