+1 on the Retro. Usualy the retros are cheaper. If you get t5 HO's id say at least 4 bulbs would do the trick. This will limit you on what you can keep due to the depth of the tank. Id sit the nems mid tank on some rock. the chances are they will like it there and not move a whole lot. Depending on what type of ballast your looking at getting, general HO ballast are in the range of 100-150 bucks. Now i believe IceCaps ballast will run 3 x 6 foot bulbs. This would be a lot less lighting but you could always add a IceCap 430 ballast to run 1 or 2 more bulbs. 3 bulbs would probably allow you keep the nems, but its hard to say. softies and leathers would be fine in that amount of light, and some hardy LPS would make it. I would not see SPS working in that amount of light.
There are a few fixtures that are cost effective and allow you to play with the number of bulbs. Current extreme pro would be a good way to go, but your starting to get up there in price. Especially when it comes to buying 6 foot setups. Currents 72 inch nova extreme is in the ballpark of 700 bucks. I would do a bit of shoping to see what your interested in and what you can afford, and play with some retro setups. Geismann also makes good ballasts, but i dont know a whole deal on there specs so i dont know how many bulbs you could run per ballast.