Ok, option #4:
Used Mamiya C330F or C330S, 80mm lens and 180mm lens. These wonderful cameras are plentiful, well made and have interchangable lenses. The best thing about this TLR is that you look down onto the ground glass and it slows you down (read: better photos).
You can probably get all of the above for $600.
I've never been attracted to the 645 format. It shows some improvement over 35mm, but if one has a 35mm system, I think a better move is to go to 6x6 (if you like the square) or 6x7 if you prefer a rectangular format. You get more bang for the buck and the bulk at 6x6 or larger. Even if you don't print square (though I usually do) from 6x6, you get the option of cropping from anywhere in the frame, so if you take the image from the top of the frame, it's like using front rise on a view camera, and if you crop a vertical from the left or right, it's like getting shift.
I would only look into 645 if I were going to have one system that had to be a compromise between 35mm and a larger medium format. The one possible exception might be the Bronica 645RF, if I wanted a compact rangefinder with interchangeable lenses.
If you're not shooting landscapes because of limitations of 35mm, don't generally think wide, and miss them enough, then a 2x3 century graphic, with some modern 6x7 or 6x9 back would be a decent compromise between negative size and portability.
If you shoot enough fashion to be frequently borrowing the RZ, then look for an older RB67/67Pro, and a second back. Your friend will appreciate it, and you won't have the scheduling issues.
Alternately, from where I'm sitting (upstate NY in the dark and damp), I'd take a week off and go somewhere photogenic to recharge, taking the Canon and 2 most commonly used lenses. (because you never know when you're going to drop one)
Go with a new format. You will take different pictures and you might surprise yourself.