Honestly doing a 29 gallon is better than a 20 gallon any amount of water volume you can add will make for a more stable tank. If you are looking for a way to save money in the long run don't cut corners because you are going to upgrade so you can do more.
If you use an aquaclear filter make it into a fuge it isn't that hard and makes it more like a hang on the back sump if you use a 110.
Light you could do a lot with a fishneedit light you will want 4 tubes get 2 blue and 2 white 2Ft/4Bulbs fixture
You will want to upgrade the bulbs when the time comes to change them out. It will make a big difference but for starting out the bulbs they come with aren't bad.
For sand don't use crushed coral that is a nitrate factory.
Two very important things no one should do without is a refractometer and a good test kit like API.
Remember if you take your time and do it right the first time it will make it a lot more fun.
If there is a local forum that is a great place to find the best deals it seems like someone near me is either taking down a tank, downgrading or upgrading. Check craigslist as well.