OK Ben, since no-one's jumped in on this I will!
I don't definitively know the legal complications, but I can't see why you would need a Q plate - you're not changing enough of the vehicle, and you're keeping the original chassis and body. Nor do I think you're changing enough to require SVA / IVA. As it's a coach there aren't any implications for plated weights, so I don't think that would be an issue. Your insurer will want to know about the modifications and may want an independent engineer's report to satisfy themselves that the work has been done to a satisfactory standard.
Before going ahead, measure up as much as possible to see how it fits; width of spring mounts, width of axles, length of Bedford vs. Iveco steering arms, axle ratios, how will it bolt together, how will the prop fit, plumbing issues, how will the steering fit, will you use the Iveco or Bedford springs, and so on.
I assume you're thinking of using the whole Iveco brake system including master cylinder and foot valve, so that'll get you round any potential mismatch, but you need to think about how you're going to mount it all and plumb it.
Given that it'll be a restored and presumably low mileage vehicle, how important will spares availability be? Are you giving yourself a lot of trouble for a purely theoretical advantage?
If it was me, I'd stick with the Bedford bits unless I had very good reasons not to. There's no real reason why a J2 should be expensive to put brakes on, and you could still convert to air over using later TK or TL bits if you wanted. The job would be fairly straightforward and quick, and wouldn't distract from the rest of the restoration.
Hope this helps