Before you start building, go shopping. Check thrift stores, Craig's list, FreeCycle, etc. Also, if you intend on building out of wood, ask. Ask on Craig's list in the tools or general section if anyone with wood working tools will help you build a hood. I know if I saw that request, I'd agree as long as I made all the cuts.
On my son's 5 gallon tank I'm using a 23 watt CF bulb in a incandescent hood someone gave away on Craig's list. There are six species of plants growing in it. The only algae is on the filter outflow which is very near the light.
For your 20 gallon, I'd still shop the free and cheap sites, but a dual T5 fixture with any type of bulbs for $22.00 is hard to argue with. As a guess, given it is a household fixture the bulbs will be 3100k to 4500k (Or possibly 10000k which some manufactures claim as daylight in T5 fixtures) which happens to be a fine place to start. Upgrade the bulbs - one at a time if you have to - later. You may find they work just fine for your needs.
If you go presurized CO2, besides blowing away your claimed tight budget status, you can add a second dual bulb fixture to your hood.
In theory a 14 watt T5 bulb should put out more light than a 40 watt T12.
In theory, theory and reality are the same.
By the way, here an idea to consider.
They can be mounted in a hood or if you can find these brackets, you can mount up to six of them on a tank.