WHen I started out...I decided to go with low light plants so as not to have to dose CO2 right away, and also to stay with the easier to keep plants at the start.
You can go about decision-making a couple of ways. Decide what kind of tank you want: low light, med light or hi light and decide what plants and equipment you need from there, or look at plants and see waht their requirements are and pick your tank style from that.
Personally...I bought a couple of plant books, and researched plants on the internet. I figured out I could find a variety of plants in the lower light/easy to grow end of the spectrum and so that's where I started. A site with an nice searchable plant database is
PlantGeek.net - Your Aquatic Plant Resource You can search by light requirements and it will also tell you plants in the same catagory.
Some choices also depend on where you whant to shop for items. Do you want to pick things up locally, or do you shop the internet? There are a lot of different sites out there, and also Ebay. You need to familiarize yourself with prices and what shipping costs are so you can decide which is the best choice.
As to your undergravel heating: I've seen books that puch it, but have never seen a "live" person that uses it. And I've seen a lot of nice tanks maintained by those "live" people.
My personal experience with substrate is using a Fluorite/sand mixture, also soilmaster select, and plain construction sand.
I prefer the fluorite/sand mix, it's more cost effective than plain flourite, and provides some nutrient value to plants that draw nutrients from their roots. Soilmaster select does somewhat, but I find that it's soft, and have a hard time initially to get the plants to stay down in it. Once they are rooted, they do well thought. Construction sand is messy and the one small tank I have with it I'm considering ripping up. If you use sand at all, use pool filter sand. It's much cleaner.
Lighting? Whatever the cheapest for the wattage of light I need for the tank.
CO2? If you start out low light you don't need to go there, you could dose Flourish Excel, which is a liquid form of biovailable carbon. You can dose CO2 into low light tanks...I haven't made that leap myself yet, but plan to. From what I've read, they can benefit from it also.
Anything else you need to know: Do you know where you plan to get plants? Most LFS's don't have much, at least around where I live. Fellow hobbyists are a great resource, I buy quite a few off Ebay, and aquabid is another online source.
DO you plan to have fish/inverts in this tank? Those will also have some effect on what you plant. Some fish just root up plants and you cant keep thiem in planted tanks.
HTH