Like already said, 9 out of 10 photos will be horizontal (landscape) so if you like the WLF stick with it, save for a prism (metered or not). I have both with my Mamiya and they both have their virtues. Congrats on your new camera!
I had both a M645 and the 1000S models loved the 1000S over the M but back to the point. Travel light, wlf is good for that. Mine had fold up speed frames on the wlf set for the 80mm. Set focus fold dowm the wlf and fold up the wires, mostly I just cropped the imaes.
Later I found the clip on mask for the speed frame wires showing the 50, 80 and150 frame lines. It was about the size of a 35mm slide and just clipped on.
Sold both riggs to KEH at a photo show, they told me the wlf w/ the frame lines would bring more than the camera bodies. I then purchased a 500cm and have spent way to much. I miss my 1000s.
I agree with most here, keep your WLF and have fun on your bike trip. While on your trip keep track of how many vertical shots you take. This will tell you if you need to save up for the prism or not. You will never regret having both the WLF and the prism. Later, if you find you make a large number of vertical shots, then plan on packing the prism with you.
I aslo have problems using my 645 vertically, so I simply use it horizontally and crop the photo later. Yes, I lose much of the negative, but I generally am doing this for me and not a paying customer. I am willing to live with that.