I would also agree that starting with a 1-2.5 gallon kit is a fantastic idea. Low cost, more managable, and you won't have 2 cases of crappy beer sitting around if you mess up. I would shutter to think of what I would do with 5 gallons when I first started experimenting with original recipes.
Start small, make sure you truly enjoy it, then figure out where you want to go. It would also help to brew with others to see their equipment and if you want to go there.
EDIT- If it helps...I started with 1 gallon kits and am still doing 1 gallon, just my own recipies. I plan on going to 2.5/3 gallons sometime soon but I almost went out and spent hundreds on 5 gallons when I am realizing more and more than I enjoy stovetop brewing and small/easy to handle/manage batches.