You could easily swap the crank from the 520 onto the 610, but if you like the fit of the 520 better I think that trumps everything else. Are they different sizes?

I'd probably stick with the 520 for the purposes you describe. Do you have any 700c wheels you can use to experiment with the brakes? That would be the first thing I'd figure out. I think there's a very good chance it will work, but it will depend on the brakes you end up with. Since you're looking at having a new front wheel (at least) built, you could stick with 27" if you had to, but it's always nice to standardize.

You shouldn't have a problem putting modern (10/11 speed) parts on the bike except that you'll need to update the rear spacing as bmthom.gis suggested (not all that hard but it might be worth having a bike shop do it to get the alignment just right). An alternative to the donor bike route is to order from one of the UK websites that sells Shimano components much cheaper than you can find them in the U.S. You can get a full 11-speed Ultegra groupset (shifters, derailleurs, crank, bottom bracket, chain, cassette, plus caliper brakes that you can re-sell) for about $550.