I did a 6 bulb T5 retro on my 125 and staggered the bulbs to cover the ends a bit better...very happy with it...I did Advance ballast (standard HO, not overdriving), Tek reflectors and ATI bulbs...I get as much PAR as 250W MH's with 14K bulbs...and I keep a Maxima clam on at the bottom of the tank with excellent growth...in Feb of this year it cost me about $470, so right in line with your budget...no heat issues at all, but my sump is in the basement which helps with cooling a bit...but otherwise my lights are enclosed in a pretty short canopy with no fans...and I use about half as much at electricity over the 3 x 240W MH before you even factor in a chiller...the only thing I miss is the shimmer.
One other note on the 2 x 36" fixtures...you'll eat up any savings in bulbs...it seems all the "better" brand T5's are about the same in cost regardless of size...so your looking at 12 bulbs to replace instead of 6 or double the cost.
...and one other final thought...depending on what you want to keep you might be able to get away with a 6 x 4" setup with staggered bulbs too.
Here is my tank with my current bulb setup...getting ready to change it up in a few weeks:
