As I get older, weight becomes a significant factor in the kit that I carry. I can't carry a loaded 20 pound camera bag, like I used to do when I was in college. I have to be selective in putting together my carry kit, to keep the weight down. It is almost like going back in time to a TLR, with just it's 80mm fixed lens, which happens to be next on my list. In that respect, I would go for 645 for the lighter weight. But that is me, at this point in my life. For me, a RB/RZ-67 would be a studio camera.
As for enlargers, why stop at 6x7? If you are considering a 23c, I would instead keep an eye out for 4x5 enlargers. Then you have the flexibility to print anything up to 4x5, including 6x9. And you will not be looking for a 4x5 enlarger later, after you outgrow the 6x7 enlarger. To keep the cost down, just get the lenses for the formats you shoot. You don't have to get the 4x5 lens until and if you shoot 4x5.
This is the route that I took, with a Durst L-1000. Now, I just need to get my darkroom built.
I've seen 4x5 enlargers (including lenses) on Craig's List going for less than $100 and even a few for free. You have to watch every day and be patient, then JUMP when you see it. Just make sure that you get ALL the parts for the enlarger. Spare parts can be difficult to find and expensive, so get the enlarger as complete as you can. In the US, parts for some enlargers like DURST, can be very hard to find, and very expensive when you do find it. I learned this the hard way, when I had to get the missing parts for my Durst L-1000. I had to buy some of my parts from sellers in Europe. Durst is out of the enlarger business, so there is no factory to get spare parts from. Omega and Bessler enlargers and parts are much more available in the US, but the bits and pieces can get expensive. Again, get the enlarger as complete as you can.