Hi Christopher,
Allow me to add my 2 cents (or 2 nickels, as I see you're in Canada). I think you might need to adjust your specifications or buy two cameras to get everything you're looking for.
I'm on my third MF camera (though I'd probably still be on my second had my cat not pulled the second one off of the coffee table).
My first MF camera was a Yashica Mat 124G. This was many years ago when MF prices were higher and it was one of the few options that fit my budget. I didn't do my own developing at the time, so I would drop my colour film off at a lab and get back the negatives and some little 4x4 inch prints. Needless to say, being used to 4x6 inch prints from 35mm, I wasn't too impressed with the little square proofs. When I did get enlargements, I found it difficult to find square frames. On the plus side, the pictures were nice and sharp and the camera was light and portable, and people seemed less intimidated by a TLR than an MF SLR. I found it difficult to adjust to the laterally-reversed image in the viewfinder, particularly when moving the camera. Being used to taking photos at eye-level, I often found myself standing on my toes trying to look into the waist-level finder while raising the camera as high as I could. I guess that's why they call it a waist-level finder. Ultimately, I found the Yashica too limiting and sold it.
My second MF camera is a Mamiya M645 1000S. It was more expensive, but since it was modular, I was able to buy it in parts. Buying it turned out to be a bit of a shopping nightmare. The film insert I bought didn't work, so I exchanged it the next day. Within less than a week, the focus ring on the lens seized-up. I returned it and got a refund and bought another. The meter in the prism never worked, and after two lengthy repair attempts, I got a refund and bought a non-metered prism. After all that I discovered that the camera had an intermittent light leak. Despite all that, I still liked it better and got better photos than with the Yashica. A set of extension tubes allowed me to take close up photos and an auto winding grip gave a more 35mm-like handling to the camera. On the minus side, it was heavy. I bought a special strap to make it more comfortable, but I don't think around the neck is necessarily the ideal way to carry an MF SLR. Being a 645 camera, it gave me 15 shots per roll instead of 12, and more acceptable 4x5 inch proofs from the lab. (Though I understand some other 645 cameras give you 16 shots per roll.) Unfortunately, I left it with the strap hanging off the coffee table one day, and our new cat, who liked to play with any string-like object, was seen scurrying away after a loud crash to the floor and I found the camera and prism separated on the floor and they would not snap back together.
Rather than getting the 1000S repaired, I decided to replace it with a newer M645 Pro TL, which had come down in price dramatically since I had bought the 1000S. I decided to go with another Mamiya since I would be able to use my existing lenses and extension tubes. The most exciting features for me were the interchangeable film backs and the (working) metered prism. One downside is that the flash shoe is on the side of the camera, unlike the 1000S, which had a flash shoe on top of the prism, 35mm-style, so you'll need either a bracket with a flash shoe, or a handle mount flash. You also need either a special adaptor or a certain auto-winding grip to be able to use a screw-in cable release. Other than those issues, I'm very happy with the M645 Pro TL.
When I bought my first 35mm camera, the advice I read recommended getting a 50mm lens because it would give sharper pictures than a 'kit zoom'. I took that advice and have never regretted buying that wonderfully sharp 50mm lens, but after 6 months, I found myself wanting a zoom lens really really badly. Quality speaks for itself, of course, but I think there is definitely something to be said for versatility. What good is it to have the sharpest lens if you can't get the picture you want with it? If you can only afford one camera right now, I would suggest starting with one that is versatile enough to take all of the kinds of photos you want to capture.
The most important piece of advice I can give from my experiences is to thoroughly check all the equipment you buy right away, so that you can return anything that isn't working. All three of the MF cameras I have bought have had issues. The Yashica had a badly corroded battery in it, which the store cleaned out for me. The first M645 Pro TL body I ordered would not couple with the aperture ring on my lenses. I exchanged it. One of the film backs I ordered with the M645 Pro TL does not reset the film counter. Unfortunately, I did not discover this until after the warranty period expired.
Thanks for reading my story and I hope there are some useful bits of information in there for you.
If I was in your position, I think I would go with two cameras. An MF SLR for general picture taking and an automated rangefinder with a built-in flash or hotshoe, if you insist on using MF for family travel photos.
If you try a waist-level finder and decide you want to go that way, it might be best to choose a square format camera as viewing a vertical shot through a waist level finder when the camera is turned on it's side will be awkward.
