One thing you could do is to start out low tech, and work your way up to high tech. High tech requires a lot of light, but a lot of light on a low tech tank could cause algae problems. One way you could do it is to buy a multi-lamp fixture and only use one or two bulbs, and then when you decide to start injecting CO2, screw in the other bulbs.
WPG is a good basic estimator, however if you have the time, I would read up more about lighting, color temperature, and PAR. This is a good article: Aquarium Lighting by Carl Strohmeyer
As for the "items" list or grocery list for a low tech tank and a high tech tank, the only different in equipment is that a high tech tank would have more lighting and a CO2 system, and you may have to supplement your plants with more fertilizer. I'm not sure what the price difference is between low tech and high tech since I have never done high tech before. If you want to learn more about CO2, go to the Equipment forum and the people there can help.
Now for your original questions:
What light do I need for a 29 gallon aquarium?
As I mentioned before, you may want a fixture that supports multiple bulbs so you can add lighting to your tank if you start to inject CO2. T5's are fairly economical and energy efficient. LED's are even better, as they run very cool and use a lot let energy, but not all LED's are made the same, and although they may put out a lot of lumens (visible light), not all of that light is useful to plants. Research everything BEFORE you buy.
What substrate should I get?
It depends on your preference, but I would highly recommend staying away from gravel. A lot of people like using soil substrates, as they contain natural fertilizers, however soil is messy to use and may be topped with sand or gravel. Commercially available substrates that, although not fertile, are as easy to use as sand or gravel but are also a good planting medium. I personally have sand, but when I update my tank, I will probably use peat moss covered with sand. There is a good article about aquatic sediments here: Overview of Anaerobic wetland soils
Are there any fish that I can't have? I mainly just want tetras, maybe guaramis, and guppies.
You should only buy fish that are compatible with each other. When you buy a fish from an LFS, ask the people there about the fish: is the fish shy? Is it aggressive? Do they school or are they territorial? Usually what I do is do research on what fish I want (or if I see a fish I like in the store) and find out the habits of the fish, then decided whether or not if it would be a good match for my existing fish. Also a note on guppies: either buy all males or be prepared to deal with the babies! They breed like rabbits.
What filter will I need for a 29 gallon aquarium?
I think a sump, as you suggested, is overkill, and for planted tanks a lot of people don't realize how unnecessary filtration is for a planted tank. Activated carbon can sometimes filter out all that good junk that plants like to eat. However having a little carbon helps with keeping your water clear and absorbs tannins from driftwood and so on. The most common filters are hang-on-the-back (HOB) filters and canister filters. I often hear people say that Aquaclear makes the best HOB filters, so I bought one for my 10 gallon and I love it. Also Eheim tends to be the most popular canister filter, being both reliable and easy/cheap to maintain. The big killer about filters is buying filter media, but both Eheim and Aquaclear's filter media lasts quite a long time.
Will I need a CO2 system?
Maybe, maybe not.
I would recommend that you read this article: Non CO2 methods by Tom Barr
What supplements are needed?
This is a really broad question that I think you should do a little research on. Do you mean fertilizers? If so, here's an article that may be helpful in understand plant's needs: Fertilizer routines, which one? by Tom Barr
I understand that this is a lot of information being thrown at you, but be patient and don't give up.
It takes a while to accumulate and finally be able to put two and two together and then, hopefully, turn theory to practice. Don't be afraid to keep asking questions, too!