Folks have been adding multi-gear hubs to single speed bikes for a long time. In my experience it won't be at all "clunky" and the only change to feel to the bike will be for the better.

Is that NOS on the old ball locking XRF5 Summit 5-speed (225%) or the new, current production XRF5(W) (256%)? The Sturmey blog guy said the new 5(W) hubs have different cable pulls than S-A's several older 5-speed designs and the new bar end/thumb/downtube shifters won't work with the old hubs.

Persons-Majestic claims to still have NOS Summit 5-speeds and NOS on the even older Sprinter design 5-speeds.

Some drum and coaster brake models of the current production (W) hub are in stock at Sturmey distributor UBS and any LBS can order from them. Among other net retailers, Biketoolsetc.com has these available.

UK net retailer Old Bike Trader catalogs several different current production Sturmey 5-speed hubs. Not cheap.

SJS Cycles in the UK claims to even have inventory of the old StAr 5-speeds, Sturmey's shortest lived design ever!

The very nice SRAM P5 hub is widely available in N.A. but they only offer a twist shifter for it - this can be placed on a Hubbub bar end adaptor.

Of course Sturmey 3-speed hubs are widely available, with a smorgasbord of shifter choices.

The other narrow ( <120mm OLD) multigear hub is the Sturmey XRF8(W) (example). Jtek offers a drop-bar end shifter for this hub. You'd probably need the companion FCS80 crankset Sturmey makes for this hub - it's not expensive.

HTH,
tcs