The Fuji GA645 would be my first choice for you. It will allow you to have fun with medium format without a weight or size penalty that you can't live with. If you have never worked with medium format you will be stunned with what 645 can do, regardless of what others try to tell you. This will probably run around $600 though I have not priced them lately. Will accept 120 or 220 film.

A Fuji GF670 (or the Bessa equivalent) is by far your best choice for a modern folder. This Fuji really is in a class by itself. A beautiful lens, compact size, excellent rangefinder focusing, a great metering system. Even the best of the older folders just cannot compete, though some come close. But, it will run $1200 for a used one, $1700 for new, though again prices can vary on this. Size and weight is similar to the Fuji GA 645 but the film size increases to 6x6 or 6x7 on 120 or 220.

A TLR is a great option as well. There are lots of options here, Rolleiflex is considered at the top of the heap, but the Minolta AutoCord or YashicaMat is also a good option. I personally like the YashicaMat, mine takes 6x6 pictures on 120 or 220 film and has an awesome lens. The film advance is allegedly not as robust as the one found in the Rolleiflex, but it has not failed me yet. The camera is light, compact, and very easy to handle. My YashicaMat cost me about $200, so it is one of the least expensive options here. However, if you have never used a TLR then there will certainly be a learning curve. I has taken me over a year to get comfortable using it.

Good luck and have fun. Medium format is still a terrific way to capture magnificent images.