In fist-time planning stages it can seem more of a challenge than it really is. Think first about the land/water ratio that you would prefer. If you put alot of forethought into it, every aspect can improve the ecosystem. With the size you are thinking about, a DIY build can allow you to be quite flexible with consctruction and design. If your set on using a tank, you still have lots of creative options. A straight-forward option for filtration would be to simply use a small canister filter. A more creative method would be to use a small pump to push water up the land portion and create a waterfall, the waterfall is pleasing to the eye and depending on how you set it up, can be a DIY "canister like" filtration system.
IMO, the best option is to set up a sump system below your tank. It allows for easier maintenance and takes all other gear(heaters, etc.) out of the tank itself.
The toughest part for me is considering filtration of the water while still having an eye-pleasing design, which is why I suggest all those different options.
A common method for creating land- use a layer of hydroballs up to the water level. Lay down a screen to seperate the hydro-balls from the terrarium substrate.
Really that is the basics all summed up. Decor can easily be incorprated to hide any mechanical parts and the water portion will provide humidity for the amphibians. Think about different options, look around for different pictures of ones already set-up and get your heart set on livestock. There are lots of great "nano" fish species for cold, cool, and warm water tanks, so I would really select your amphibian life and arrange the tank around them. In many cases its hard to mix various amphibians in one enclosure, especially if your going to use only a 30. One thing to consider is how they change throughout their lives. Axolotls, for example, can be cannibals when babies, but tend to ignore anything that wont fit in their mouths once they have grown.The fun is designing and setting up the decor, so dont let us steal the fun from you!
If your really thinking about dart frogs, look up black jungle terrarium supply. They specialize in dart frogs and terrarium design. Very helpful, knowledgable and a good line of products. Ive been to their facility multiple times, they breed their frogs and all of the adults are set-up in customized terrariums