You need an overflow (box or pvs pipe/can be homemade), some plumbing...and whatever size tank, like a 30 gallon tank ($30 for a 30 gallon visio glass tank). Then you buy pre-cut sheets of acrylic/polycarbonate (or you can cut them yourself) and just silicone them in however you want. A DIY sump <-- would cost about $50 for a 30 gallon, I'd say. $100 max for a larger size. However if you're ordering a pre-made sump they usually run about $250-500 depending. To me, that's outrageous! I have a 55 gallon tall sump b/c the dimensions of my tank are strange and I wanted a taller sump (usually sumps are only 17"-19" and I use an external pump. If you have an internal one, then account for that. Also, if you get a protein skimmer, make sure it will fit in the sump! Your husband could no doubt make one! You can watch youtube videos of how people did theirs, or look some designs online.
If it was used for FW make sure to clean it out really good. The waste will cause problems with the parameters. Dwarf angels are awesome, especially the flame ones! I wanted one, but they can nip at corals and are semi-aggressive and that wouldn't work since I have only peaceful fish and am going to get corals. They would be fine in a FOWLR, spark interest and vibrant colors!
I know how it feels to have lfs sales all the time and I'm so tempted, but...must...have..self..CONTROL!! haha. You can use T8, T5, Power Compact, anything really works. As long as you see the fish you'll be good (this is for FOWLR)
!
For a cheaper option you could buy some BRS pukani dry rock. It seems expensive, but because it is so porous you get lots of pieces for less money than you would with dense LR (that is soaked in water). Plus I personally love the appearance more than usual LR that is usually Fiji rock. I bought 100 lbs for my 70 gallon and had over a quarter left and got 12 one foot pieces, and 3 two foot pieces! Plus no pests!
BRS Pukani Dry Aquarium Live Rock - Bulk Reef Supply Other good dry rock I heard was from MarcoRocks.
And btw you don't need really expensive lighting for simple corals like mushrooms and most soft corals. PC and T5s do okay or try check craigslist for a bargain!
And for your daughter, how convient!:
The Nemo Tank Guide - Aquarium Advice