I was in the same position as you before Xmas. Used SPD-SL on the road bike, but had always used flats off-road.
I went for the M530s as I managed to get them for the same price as the M520s. I think the extra platform does help clipping in/locating the right point, but if the M520s were cheaper, I don’t think you would do any harm trying them.
As for shoes, I got some Shimano XC50N. Again, I paid nowhere near the £80 RRP, as got them in the sales, but went for those due to the weather in this country of late, as they have no vents. Will probably be too hot in summer, but dusty trails are a pipe dream at the moment 😥
In my hunt for cheaper shoes, as like you didn’t want to commit too much, and tried loads. All different brands seemed to have differing fits which was a pain, but it’s where Wiggle/CRC free returns comes in handy. If your budget is around £30-£40, then Wiggle’s DHB offerings were quite good for the price.
http://www.wiggle.co.uk/dhb-m10-mountain-bike-cycling-shoe/
Easy to walk in, not too narrow and look ok in black. They also do a carbon soled version for £60-ish quid which was nice and light: http://www.wiggle.co.uk/dhb-m20c-carbon-mountain-bike-cycling-shoe/.
Other than that, if going for Shimano shoes, they do seem to size quite narrow, but you can get wider fitting version….or at least you could in some models.