Hi, err..The Plant Man With Rams, welcome to the forum. We're thrilled that you chose a planted tank because the mbuna are so obnoxious. Much better to have a nice green work of art than a tank full of quarrelsome fish.
The bad news is that a 100G tank, properly done demands a substantial investment but there are some compromises that can save you money.
Plants - Telling you what plants you need would be like telling you what Rams you need, or what color shirt you need, or even what brand of beer you need to drink. The field is wide open and you have a gamut of choices. Look around the forum and get some ideas of what plants you would like to have. Of course, some plants are easier than others, so if I had to say which ones you "need" I'd have to say you "need" to start with some of the easier ones until you get a handle on how to grow them. I always like to point people towards the Tropica website
http://www.tropica.com/default.asp
This is a good reference for determining the relative ease of a plant, it's demands and what it looks like, more or less.
Substrate - Plants can be grown successfully with high powered substrates such as the ones sold here:
http://www.aquaessentials.co.uk/index.p ... 69989a1fa1
You can see that the most expensive substrate (ADA Aquasoil) sells for almost £30 for a 9 liter sack and for your tank you'd "need" maybe 10 sacks. On the other hand you could go to the nearest playground and nick an equal amount of sand from the play box for free and be about as successful. Somewhere in the middle of these two extremes is a compromise of lower cost substrates. The visual aspect is as important so choose the colors and textures that you like. You can also mix and match without penalty.
CO2 - This page shows a couple of regulators and solenoids
http://www.aquaessentials.co.uk/index.p ... th=194_251
They are fairly expensive but you can also buy a complete unit from Malaysia from ebay sellers such as this:
http://cgi.ebay.com/CO2-Regulator-Elect ... m153.l1262
You will also need diffusers. Both of the shops I gave the link for sell diffusers. High brow->
http://www.aquaessentials.co.uk/index.p ... th=196_197 http://www.aquaessentials.co.uk/index.p ... ts_id=1694
Low brow - >
http://stores.ebay.com/AquaticMagic_CO2 ... idZ1QQtZkm
You then have to figure out where you will get your CO2 cylinder and re-fills.
I assume you already have a canopy and appropriate lighting?
I also assume you have sufficient filtration, somewhere on the order of 1000 Gallons per hour turnover rating? If not you'll "need" to add more filtration. Two Fluval FX5 would do nicely on that tank for example. Biomedia to fill them as well.
Chew on these figures for a while and we'll answer whatever additional questions you have.
Cheers,