Here are my quick 2 € cents:
- This camera screams for a tripod. Yes, you can use it hand held and some folks seem to enjoy it that way, but given that you already have the GW690 I suggest you go fetch a sturdy tripod when you use your RZ67. Also fetch a cable release.
- When you buy used lenses or accessories, make sure you get parts which work with your camera. I've seen lots of RB67 lenses/accessories incorrectly labeled as RZ67 parts. Some RB67 parts work with the RZ67 but some do not.
- I agree with Jeffs call for getting multiple film backs. I have 4 now and sometimes wish for more
- Read the camera manual carefully before you go out shooting. The camera can do lots of tricks but it's sometimes not obvious how to make it do a particular trick (check battery, mirror lock up, long time exposures, ...)