I'll offer a contrary voice to Adam. I shoot my MF 99% of the time and my digital takes a back seat.

But that's because I'm a BW shooter and prefer the look of it in film. Your milage will vary.
TLRs are a great camera and fun to shoot. But not having interchangeable lenses can be a constraint. Since it won't be your only camera most likely that won't be a problem. My Rolleicord doesn't get used too often because of that. But when I what to carry hardly anything and just have a camera along, it is a good choice.
Waist level shooting is a personal preference. I find it hard to tell someone if they will like it. And if that Ricoh copies Rolleis, you'll get an option of eye-level composition and shooting. My personal preference is for waist level because I like a 100% view and the reduced size and weight it offers for my Pentax 67. But the TTL prism will go on it when needed. My Hasselblad has a WLF too. So I guess I do a lot of shooting in that mode.
The economics of purchasing a 645 and assorted lenses depends on how much you will use it. But note it seems the 645D has everyone in a frenzy buying up the used lenses for this camera.
There are a lot of 645 shooters here and they can offer you some fine advise on that system.