Your plan sounds completely reasonable.
I personally dont like undergravel filters because 1) they make having live plants extra difficult. 2) there isnt as much surface area for the bacteria to grow on so this in my mind makes them less efficient. I'm not sure how much you know about the nitrogen cycle but I'll give you a run down in case (if you already know I apologise). When fish produce waste it is pretty much straight ammonia. This can kill them easily. The first type of bacteria then starts to grow in the filter which eats the ammonia and excretes Nitrites which are also pretty toxic. Then the second type of bacteria starts to grow which eats the Nitrites and converts them to Nitrates which are much less toxic. When this cycle is starting up you will see the Ammonia grow till it spikes and starts to fall, when the Ammonia is almost at its peak you will see the Nitrites increase till they too spike and decrease. As the Nitrites are almost at thier peak you will see the Nitrates start to rise. When the cycle is complete you will have a reading of Ammonia: 0, Nitrites: 0 and Nitrates: anywhere from 20 ppm upwards I think. In a tank that runs well and is not overstocked you do the water changes to keep the Nitrates down. Filter media such as sponges or ceramic noodles have a much greater surface area on which the bacteria can grown meaning the filter will be able to cope with alot more waste. You can try getting ceramic substrate to put under your pebbles on top of the filter plate and that may give your bacteria more surface area to grow on. You will still have trouble with your live plants though.
As for the water conditioner, I wouldn't stress, everyone has thier favourite brands and some have things in them that will help fortify a stressed fish but so long as it takes out the chlorine and chloramines then its doing its job and it will be fine. I use the cheap stuff all the time (with 5 fish tanks in the house we use too much to get fancy). Just out of curiosity, what is the pH of your tap water?