The 35mm is why I bought (another) M645. I'm a Hasselblad user and I could not justify the $$$ for a 40mm 'Blad lens or the SW/M with the 38mm. So I bought the 35mm and a 1000s body. It's my 3rd Mamiay 645. I really like the 35mm lens!!! Not very heavy and is tack sharp. I bought it for travel shots.
With the 80 a lot of folks get the 45mm.
You might find the 35 too wide - think a 21mm on a 35mm camera. But with the 55mm it's a great set up.
Yes, the 45mm would a lot cheaper, also. Good idea. I will try the 45mm.