The Pentax 67 or Mamiya RZ would be where I would start looking for 67 (I have the RZ), or check out the Hassy 501/503 etc for 6x6, although there are a ton of really great options for both out there.
A bigger negative will give you better scans and better prints, just because there is much more information to be had. The difference is immediately apparent in the darkroom because that process doesn't have all the variability you can get with digital--choose your format, choose your iso, and choose your enlargement, and that dictates the quality of your print.
If you're seriously getting into film, it will definitely be worthwhile to get your own scanner. I'm not super up-to-date with what's great out there right now, but I have an Epson v700 and it works well. In the past, I've made prints up to 24" off of scans from it and they've looked good. Scanners are kind of a big investment to make right off the bat, but if you take into consideration how much it costs to get a decent scan somewhere else, and then think of how often you'll be shooting film, you should be able to figure out if it's worthwhile.
Once you start shooting 4x5, you'll quickly realize it's not a good format at all for hand-held shooting. The rangefinders you see on old cameras are from way back when they were used as press cameras, so if you see that, it's an old camera. For a starter camera, I'd just find something like a Graflex Speed Graphic, make sure the body and bellows don't have any light leaks, and get a lens somewhere in the 150-210mm range. Once you figure out the ins and outs of shooting with it, you'll never look back--nothing can replicate the image quality of large format!