If you go planted, make sure you research if the fish are good with plants or not. I LOVE planted tanks, I think they're gorgeous. I'm going planted with my 100 gallon after I sell my Geophagus Altifrons(google it - beautiful fish).
Bigger is definitely better when it comes to aquariums, a larger aquarium will be much more stable than a small one, giving you more room for error. I have been keeping fish in my 100 for years now & have only had 2? deaths, and this was from buying fish that weren't in great condition. Lesson learned.
If you do get plants for your tank you'll want to make sure you have good lighting, and research each plant species you want to get to make sure they'll be good with your amount of light. Start with low-medium lighting and if you get addicted to plants like I have, you can upgrade from there.
A 100 gallon tank will cost a fair amount to start up, but once you've got it up and running the maintenance costs are minimal. I bought my 100 gallon off of Craigslist for $300, and this included the tank, stand, canopy, filter and heater. Keep your eye out for good deals on Craigslist, they pop up quite often. I got a fantastic deal. Depending on your stockload(the amount of fish you were to keep in the tank) on a 100 you could run a filter that is anywhere from the Aqua Clear 110 (for light stockloads - about $60 online) to the Fluval FX5 (what I have since I keep a LOT of fish in my tank - runs about $250 - $300 online). My heater cost me about $40 for a good one. You can get white Pool Filter Sand or black Sand Blasting Sand to use in the bottom of the tank, both work fine for the vast majority of fish, just make sure you rinse them out VERY well to get rid of all the dust first, otherwise your tank will be very cloudy. Both are cheap, only a few bucks for a 50 pound bag. I prefer black sand as it makes the colors of the fish pop, but some people prefer white. Overall, my 100 gallon setup cost me about $700 including my expensive filter & new lights. You could do it for several hundred dollars cheaper if you keep a watch out for good deals on equipment & tanks.
Filtration is very important and isn't something you can skimp out on unless you want your fish to die on you. You need to make sure you have adequate filtration for the number of fish you have, we can help point you in the right direction for a filter after you've decided on what you want to put in the tank.
For suggestions for fish, a couple things would help us decide what you might like:
Do you want peaceful fish, or aggressive fish that will eat smaller ones?
Big fish, or small fish?
EDIT: By the way, if you buy a used tank...make sure you see it filled up & holding water! Very important...some people will try to sell you a cracked or leaking tank, so if they won't let you see it filled up to the rim & holding water, don't buy it.
EDIT AGAIN: Here's my 100 gallon about 11 months ago.
http://www.youtube.com/watch?v=4dTLYCOV1MQ