Go for large tyres and some mudguards, for now. Some good mudguards in steel/ally or perhaps white ones would be great!
If you fancied using 27"s, the next step would be to start going to jumbles, to build some really nice 'period' wheels. I have to say there's something rather lovely about using 27" wheels, the slightly bigger size is very pleasing to ride.
Look out for Airlites as these are the best high quality old hubs you can find easily enough, for not too much dosh. I know of one pair for sale (not me), let me know if you want to know who's got em.
Classically they come 40 hole rear, 32 hole front.
Finding a good pair of 27" rims with those drillings really is about going to jumbles. Ripley, and get yourself to all the little ones too. Look out for Weinmanns, Conloy, Rigida, even some steel Dunlops.
Don't worry about quality, the old stuff lasts forever, provided they're built properly they'll be fine (40 spokes on the rear!) Just polish the hell out of everything before you build them.
Your best bet for tyres will be Pasela Panaracer, you can find them in 27x1 1/4 pretty easily. Also Michelin do a simar one, with classic looking gumwalls. I heard you could pick these up at Wilkinsons really cheaply. The magic number to be sure you're getting the right tyre is 630, so you'll be finding 630x32 or 630x28 perhaps.
Hope this is all of interest!
Good luck with the build. What cranks you going to use?