Yeah, the 3 big ones are:
Club Car
EZGO
Yamaha
As mentioned before, Club Car's frames are aluminium. EZGO's are steel... Don't know much about yamaha...
You can choose Gas or Electric... and electrics the older models are 36v, newer ones are 48v...
Modifying a cart is extremely easy, check out
http://www.buggiesunlimited.com/ for various parts and stuff...
Usually if you can find one that already has the features you want it is cheaper than buying one and modifying it yourself, but parts aren't *too* expensive if you shop around (and check eBay)...
The E-Z-GO TXT is a very very common cart and parts are easy to come by. If I had a choice I would get a late-model used one, 48v with new or less than a year old batteries (always take a test drive and check the battery water levels and how much corrosion is around them)...
Often golf courses lease their carts for a year or two, then dealers will get them back, clean them up and sell them. So you can get good deals that way. But there are always lots of used carts out there at the big dealers, most will install fresh batteries so you have some sort of warranty. If you buy from an individual you really have to take their word on everything.
I've seen basic TXT carts going for under $2k, and decked out ones, well the sky is the limit...
If you are going to have a flip-flop seat in the back, you will definitely want heavy duty springs in the back. Usually most carts you can put up to 20" wheels without a lift. If you want 21"-22" then something like a 3"-4" lift will give you clearance.
Once you get beyond 22" tires then that starts to push the limits of the stock speed controller and you get into the realm of upgrading that and power cables, switches, solenoids, and sometimes even the motor...
Also realize there is a tradeoff, you get more speed with larger tires but you also have less torque. So if you live in a very hilly place, that is something to consider.
Here's my golf cart (2001 EZGO TXT 36v) that we take when shooting sporting clays. I recently put a 6" lift on it with 21" tires. You can really tell the reduction in torque with the larger tires but since I drive mostly on flat ground it's not a big deal. When I replace the batteries I'll probably replace the cables and various related parts, see if it helps out. Then the next thing would be to upgrade the speed controller but those can get kind of pricy.