I used to shoot an RZ. I had both the waist level finder and the metered prism finder. I loved the waist level finder but hated the metered prism finder because it adds more weight to the camera.
If you buy an RZ then get the "L" grip. It really improves the way the camera handles. It's a great system if you don't mind the size and weight. I sold mine to help fund my large format gear.
I've the RZ Pro II
there's no built in meter, it's an accessory. There are two. The first one and most common and easy to find is the AE prism and you get good one from $200 to $300. The other one which is super rare (I only saw 2) is the chimney finder and people tend to overprice it up to $500 and $600