That's a start!
You will need a pretty good light fixture to get your Dwarf Hairgrass to grow right. Depending on the size of the tank you get, a T5HO fixture should do the trick. The only problem is, if you go w/ a high wattage fixture like that, you would need to run a CO2 system or else you will have an algae bloom like you wouldn't believe!
A CO2 system is also expensive and can be too complex for beginners. Which is why I never went w/ CO2. I have a low-med light setup and dose a liquid form of CO2 called Excel. Originally I had a single T5 fixture, but I noticed that my plants were dying off. Then, last week, I added another T5 fixture and BOOYAH! they're taking off.
Planted tanks are a lot of fun, but if you're new to the hobby, it would be best to start out w/ low and move up to high. This way you can get a routine going w/ dosing fertilizers, and adjusting your photoperiod, (how much light exposure your plants will get during the day).
About your stock idea, it sounds good, except w/ the Mollies, Discus, and Cichlids.
Mollies are Brackish fish and may or may not do well in a freshwater tank. I had 3 Gold Dust Mollies in mine, and they did "ok", but they require a certain amount of salt in the water to keep them healthy. I would just substitute Mollies w/ Swordtails if you're looking at getting a nice group of livebearers.
As far as Discus, from what I hear, they can become aggressive and should be kept in a Discus only tank. The Cichlids in a community tank is a really bad idea. They are way too aggressive! Even if they don't technically "kill" any of your community fish, they could stress them out to the point where they could die.
Undergravel filters suck ass! Seriously! I had a partial UGF setup in my tank for about 4 months until I finally had it w/ the amount of crap/debris getting clogged beneath the grates. If you're looking at a tank larger than 75 gallons, I would suggest going w/ a Canister filter. Otherwise, there are some really nice Hang-On-Back filters. Just make sure that you get a filter that will accommodate your tank size.
Heaters are heaters, but just make damn sure it is made for your tank size. The best place to start would be to cruise the Petco/Petsmart sites and read reviews. While you're on the sites, you can also check out their fish and write down the names of the ones you like... then research away! Also, Petco has a compatibility table that will help guide you in your selection.
JFC! I wrote a damn novel!
Anyway, there's a lot to learn, but it sounds like you're motivated to get into the hobby. Just take things slow, research everything, let the info sink in, and never feel like an ass if you need to ask questions.