First of all, you should join Dendroboard. It is a forum for people that keep dart frogs. I've posted the link below for you.
I personally prefer the Exoterra units to the ZooMed ones. You don't have to buy the Exoterra unit as a kit. They do come as just the empty terrarium as well. I prefer to buy mine as just the empty tank, as I like to decorate the tank to my tastes.
You will probably have a hard time finding a sexed pair or a pair of proven breeders for cheap. Sexed animals are usually more expensive the unsexed juveniles.
Go to a few reptile shows to get some of the animals priced out. Also, check places like Craig's List, sometimes you can luck out and people sell their breeding stock off because they are getting out of the hobby.
If you are planning on putting live plants in your tank, you will need to set up some sort of false bottom, with either egg crate or LECA pellets. This is done to prevent the roots of the plants from rotting if they get too wet. Egg crate you can pick up at any hardware store (Home Depot) and LECA pellets can be picked up from any garden center that sells Hydroponic supplies. ZooMed also sells them, but they are really expensive for a small bag, and they are called "Hydro Balls"
Most dart frog keepers spend months to at least a year planning out and setting up the tank before actually getting the frogs. If you are on a tight budget and don't have much extra spending money each month, this can work out to your advantage.
Check out Java Moss. It is moss that is sold for aquarium fish, however it can be kept in dart frog terrariums if it is really humid and misted every day. It is a slow growing moss, and does take a while to attach itself to logs, rocks, etc, but it looks very attractive when it's done.
For soil, I like mixing the Cocofiber, Peat Moss, and Worm Castings. This makes a nice dirt that I use in my darts tank.
Dart frogs also really like Leaf Litter. Oak, Magnolia and Sea Grape are the best because they last a really long time, but other (non-toxic) leaves can also be used. In smaller tanks, like the one you are interested in, look for smaller leaves.
To clean the leaves, dry them, and bake them in the oven for 20 minutes at 250F. If they are fresh leaves, you can also give them a wash in a mild bleach solution, rinse well, dry then bake.
Be careful which plants you choose, as lots of the tropical plants do get huge.
You can get cheap plants at places like Walmart.
You can also grow your own from seeds. Mint plants are really easy to grow and look attractive in terrariums.