Seems that you don't really need AF, and as you are on a budget, you might reconsider the 503cw. Used it is quite affordable, lots of lenses and stuff on the used market. Then get a refurb back of your choice.
I shoot a 503cw with a p45+ myself from time to time and I love it. Very versatile, with winder and 45° prism if weight doesn't matter, without winder and the wlf if it does. And the V backs also work on rz67 and any large format or alpa, or whatever you might need for your wide desires, with an adaptor. If you like to tether I would recommend phase one, if shooting speed, high iso and on cam display doesn't matter a lot to you, a refurb P45 might do the trick very well.
cheers,
martin