I do not know where the cut off is, perhaps Bang Guy will chime in, but a DSB is not always considered to be terribly functional in smaller tanks, and I am not sure a 4" sand bed in a 29 would be worth the lost space.
Instead, might not be bad to consider adding a refugium.
A few basics, what I consider to be the most important stuff, and I have written it before:
You can have a lot of money, the best equipment and more gadgets then anyone, but the most important things you need are things you can't buy.
1) patience (it takes time to do it right)
2) knowledge (read, research, ask questions, read some more)
3) patience (you can't keep a lot of fish, even if you like them...)
4) discipline (to do the work, maintenance, and for #1-3)
5) patience (it takes a long time to properly stock a salt water tank, many many months or longer)
6) humility (to admit when you've done something wrong, to know when you've forgotten about 1,3 and 5, gone out, bought a fish without doing 2, and now it is dead)
7) patience (the tank might be empty or just plain ugly while it cycles, but you have to give it time to mature)
8) responsibility (to do 1-7, to face a mistake as in 6 and learn from it)
9-99) patience (money won't get you out of most problems, 1-8 will)
100) money (for when you realize your 29 is too small and want a new tank)
The best place to start is researching, and by getting a few books to decide what you want to keep in your tank. If you know anything about freshwater tanks, now is a good time to set it aside (except for how to use a siphon) and start learning about saltwater tanks. Very different indeed.
Search for things like "Live Rock (LR), Live Sand (LS), Deep Sand Beds (DSB), Cycle, Cycling, refugium" There are important terms for an early tank.
I am serious about the patience. You can have a lot of money to put into the best equipment, but without a lot of patience you will not succeed. You can have a very nice tank without a great deal of money (relatively), if you are patient. It is the golden rule of reefkeeping. IMO
Please take your time and decide what you want to keep and how to do it best.
WELCOME TO THE BOARD! Ask, ask, ask away!