Use bark. It doesn't have to be 'Repti-Bark' (they think they can charge you more if it is 'special' bark for reptiles!). Pretty much any bark chips will do, as long as they are small enough and you didn't get them from your backyard. You could check the price of orchid bark or some fine bark mulch from a landscaping supply place. They may have a minimum amount that they sell, which would be way more than you need, so call ahead. I just used cedar bark mulch from Home Depot. It was a large, 2 cubic foot bag, but it was exactly how much I needed for my 85 gallon tank. Don't use gravel, perlite, or vermiculite, because these things can be ingested by any creatures you put in your tank, and may be too large for the poor animal and 'block them up', if you know what I mean. This leads to a painful death. So use something natural. (And yes, I know rocks are natural, but not biodegradable, so stay away!) Also, organic materials will be a much better planting substrate for live plants than anything innert.

I didn't worry about fertilizers in the plants too much. Just un-pot them, carefully dust as much of the dirt away as possible (this is much easier if the dirt is nice and moist), trying not to tear too many roots (though don't be too upset if you do; plants are quite tough), and then rinse the rootball off under water for 5 minutes or so. This should flush the fertilizers from the soil, as well as extra dirt and things like perlite.

As for bog plants, it depends what kinds you want. What did you have in mind? Most bog plants like a peaty mix, so you could either keep them in pots (they are usually sold in the correct mix, if you bought them from a reputable dealer) and hide the pots with sand or pea gravel (the smallest you can find), or you can create a layer of peat moss in your water area, and plant the plants in there, provided the water is shallow enough for them to emerge from. This might make your water permanently 'dirty' though, as the frogs hop around in it and stir up the peat.

If you need any suggestions for easy plants for terrariums, search this forum. Someone named DianaMay posted a very useful list a long time ago, but the thread has not 'fallen off the edge' of the forum yet. =) I have had great success with fittonia, ferns, selaginella, broms/tillandsias, and paph. orchids. I would leave the orchids for now if you are a newbie. All the best, and have fun!