I've done this, but I think anything I have to say has already been said above. But I won't let that stop me. Here's my experience.
Brakes - you will/may need long-drop calipers (I see you've already checked this). I did and it was a bit of a faff sourcing long drop nut fit brakes. They aren't all that common.
Springing a wide hub into a just too narrow frame - yes it's entirely doable. But it's not exactly easy. Especially not at the roadside, in the rain, with a group of other people staring at you willing you to get a move on.
Cold setting, or as I prefer to call it "bending" the frame. Not difficult with a piece of threaded bar and some nuts and washers but the result isn't entirely satisfactory (at least not when done by a cack handed bodger like me) because the dropouts are no longer parallel. Don't do it if you love the frame deeply.
Compatibility -- I put an 8 speed freehub wheel onto a bike that formerly had a 5 speed freewheel. The smallest sprocket can't be engaged without jamming, so I set the Hi screw on the rear mech to keep it out of bounds.
Rear mech - I installed a rear mech with sufficient capacity and large sprocket size. Alivio I think the name was
In retrospect I don't think I'd do it again. Just too much faff. It's forcing the bike to be something it isn't. If you want something else, then get a bike that's designed that way. (Just IMHO)
Pretty much my experience. Cold set the frame, no problem.
Deep drop brakes i found lacked real braking force because of the length of the arms and (i assume) flex.
Fitting them...the Clubman i had had a tubular bridge to mount the nutted brakes and required drilling out somewhat to get them to fit. The tube isn't that big or that strong, its one area with hindsight I felt uncomfortable having done it.
Handlebars...I fitted more.modern ones because being from the 60s I think, they looked odd by modern standards and modern ones more.matched what I was used to.
Trouble is, I ended up with a bit of a Frankenstien bike aesthetically...I rode well but I was never really happy with it.
I do remember the forks and frame on the Clubman gave a silky smooth ride, fabulous for that