You can get PC
lights on ebay for about 20-30 bucks each and sometimes get a deal on a set of them. The ballast for these lights are pretty cheap and I have seen them in the 20 dollar range. Granted they are about 55w -65w but they are better than the 40 that you have now. GSPs will grow fine in PC
lighting. I had a 55g with four PCs (two white and two actinics) and they grew really well. They started as a 2 inch frag and grew to about ten inches round and completely took over the rock they were on. You definitely need more rock in there if you plan on putting GSPs and mushrooms in there. If you buy base rock you would be fine just buy yourself a good sized piece of live rock later on. Make sure the LR
is cured or has cured in the shop for at least a month before you buy it though or your tank will go threw a big cycle. Even cured rock may cause a cycle in the tank but if it cured for a while in the store and you get it straight home it's not likely to go through a major cycle.