If you know you are only going to use one roll at a time and not switch different films out, then don't worry about additional backs.....get some inserts instead. I have loaded up to 5 inserts with the same film and gone out and shot the crap out of locations and subjects. It allowed me to not waste time unloading and loading film all the while missing that potential shot that was going to be the award winner.
Of course, I do highly suggest getting another back or two.....at least one back and one or two inserts for each type of film will give you maximum flexibility if needed. Leave what you don't need for the day at the house or hotel to keep the load light. I have found it nice to be able to throw a dark side into a back, slip on another type of film and continue shooting at the right location. I mostly have used color film for 99% of what I do on my 645 Pro, but am starting to be drawn to the power of black and white.
I can't really recommend a wide lens because I haven't got one for my 645, but am considering like you to get one. I am getting away from the long lenses (in 35mm and MF) and getting more into wide (mainly with 35mm).....thankfully options are reasonable for us.