Welcome to medium format! It's very different from 35mm, and you'll probably get a kick out of trying 120mm film especially the first time you get your film back from the lab. The size is vastly larger than what you are used to!
Lots of good advice and recommendation from other CSers here... so here are tips to help you along.
1)Format
Like what Shadowleong mentioned, you have to decide which format you like first, as that determines the models of camera you can choose from. Some people are more comfortable with a square format, some with a rectangular and others love the panoramic format. In general, 645 and 6x6 square formats cameras are easy to handle, while 6x7 and panoramic cameras are usually more bulky.
2)SLR or Rangefinder
After you decided on the format, determine if you want SLR or rangefinder. Rangefinder MF cameras are compact and light, but they are not as versatile in applications and system as the SLR types. It's the same arguments between 35mm rangefinders and 35mm SLRs, except that many medium format SLRs can change film type mid-way via magazines, which rangefinders can't. Your application will determine whether a rangefinder or SLR is right for you.
3)Budget
Decide on how much you want to spend.
You mentioned you like outdoors photography, so I'm assuming you don't want anything bulky or difficult to handle. That rules out most 6x7 cameras such as the Mamiya RZ and RB, Bronica GS-1 and Pentax 67. If you like to take macro or telephoto shots, then rangefinders such as the Mamiya 6 or 7, or Bronica 645 will be out as well.
There are twin lens reflex cameras as well, which are really cool but may or may not be suitable for your application. They're light and pretty vibration-free, but you can't change lens, or interchange film half-way, and they are relatively limited for close-up portraits and macro, and they are virtually all 6x6 format.
If you're looking for a versatile camera system , you can consider the 645 medium format SLR cameras such as the Mamiya 645, Bronica ETRSi or the Pentax 645 (cannot change magazine for the Pentax though). Or if you appreciate the 6x6 aesthetics, consider the Bronica SQ-Ai or Hasselblad. All these cameras are readily available on Clubsnap classifieds for really good prices, so keep a look out!
I'm a Hasselblad V-system fan, and I've been shooting with Hasselblads for more than a decade now. I love the 6x6 format, the aesthetics of the Hasselblad design and the wide amount of lenses available. It is designed to be totally mechanical, so you need a bit of time to get used to it, but it is really easy to use and smooth once you get used to it. Some say that the system is problematic because it is prone to jamming, but personally I've never jammed a Hasselblad in more than 10 years. It's mechanical and beautifully designed, and it is probably the closest match to a Nikon FM2 in philosophy.
There is not much difference between 35mm film and 120mm film, since they behave similarly to light. You will however need to get used to the shallower depth of field, handling of the camera, loading of 120mm film and taking metering manually (if your camera does not have a built-in meter).
