If you are going to keep this pendant 12" above the water surface then I dont think that your temp will raise more than 1-2 degrees. If it does then I would install a small fan to help. As far as your evaporation I would not expect it to be more than 1/2 gal more than you have now. The only real way to find out exactly is to do it and be prepared to make any nessesary adjustment. sometimes its hard to tell until you actually do it. I will say that it surprises me though that you are evaporating 1 gal per day without MH. Again, my tank evaporates 1/2 gal per day with MH.
Every tank is different though.
The Coralife aqualight pro is a very good setup in my opinion. You say you got it for cheap, how cheap if I may ask? I paid over $700 for mine(brand new)!
-sawsman