When I first got into planted tanks back in college I right away got a decent pressurized system and I remember it being somewhat overwhelming. In reality it's actually very simple. Some things you really cannot skimp out on or you'll have issues down the road.
Here is a list of what you definitely need:
1. co2 tank (can be purchased on craigslist...go for a 5lb one first if you have room)
2. regulator (I'd get oldpunk from here to build you a cheaper one that can run at pressures of 30psi+. The reason why I prefer my regulators to be 30psi+ is that I use the GLA atomic diffusers which are great. If the regulator does not go higher than 30psi then the diffuser will not work properly. I believe GLA sells regulators that will power these diffusers too but they might be more expensive than oldpunk building you one so check).
3. bubble counter (you can buy a regulator that has one built in or attach a separate one to your co2 line going from the regulator to the diffuser). A bubble counter will help you fine tune how much co2 you are putting into your tank.
4. co2 tubing (get it from GLA since using regular airline tubing is not good...it will become brittle and disintegrate)
5. diffuser - I use the GLA Atomic diffuser. Go for the biggest one you can get since you can always lower the co2. The lower the co2 is going into the tank the smaller the bubbles. If you get a small diffuser and you try to put the co2 pressure on really high then the bubbles will be large...you want smaller bubbles since they diffuse in the water better instead of just floating to the top.
6. Hydor Koralia Circulation pump - I use one in all my tanks. It's usually pointed at the diffuser so the bubbles go all over the tank. You can be pumping a ton of co2 in the aquarium but it won't do you any good if you're not spreading it all over.
Here are some links to checkout:
hydor koralia (they come in different sizes...get a right one for your tank):
30psi+ regulator (contact him with a link to this thread and see what he can do for you):
On a side note:
You can get a really cheap regulator and just run one of those glass ceramic plate diffusers. This will get you by but the atomic diffusers are really nice. You will get 100 different opinions when it comes to co2 setups so do your research on each one...everyone has their own methods which work. Some are better than others.
Here is my current tank which runs the exact co2 setup that I've described above...it's 75 gallons with 3 foot by 3 foot dimensions: