To start with, I believe it's always better to buy new if you can afford it. There's no sense in buying someone else's problems. Also, with a new sled, you'll have the warranty that goes with it.
Let's talk displacement (cc's). This is a personal choice, but since you are just starting out, you might want to look at something in the 600 class. Anything bigger might be a bit much. All the major manufacturers (arctic cat, yamaha, ski-doo, and polaris) produce excellent sleds in the 600 class.
The type of riding you plan to do should determine the type of sled you buy (trail, mountain, etc.). As for suspension and brakes on the newer sleds, they are all very good, and all will do the job. Technology has made leaps and bounds in the snowmobile industry in just the last 5 years.
And last, don't be afraid to ask alot of people who snowmobile questions about what sled they prefer, and what to look for. Contact dealers in your area and get as much information as you can before you decide on one brand or type of sled. Once you've made your choice on the type of snowmobile, you can't go wrong with whatever brand you decide to buy!
P.S.- And welcome to the world of snowmobiling!, there is NOTHING like it!