To tell you the truth, this is quite a bit of work, it's easier to start with a mostly complete bike, and start to swap things here and there. A complete three speed can be found for 150 USD. The three speed hub can be cheerfully left in singlespeed mode, and if you are looking for a fixed gear, Sturmey Archer currently makes a fixed gear three speed hub, the S3X (pick the 120mm version).
If you insist with the frame, make sure that the seller includes the Bottom Bracket (and ideally also the cranks) as well as the headset. Since it is a frame made after Raleigh bought Hercules, it is threaded with Raleigh (26TPI) threads, which only Raleigh made. This affects two areas, the fork, and the bottom bracket.
To my knowledge, only the Hindus make Raleigh threaded headsets, and these can be found on Yellow Jersey. In order to use modern cranks, there are threadless bottom brackets available from Velo Orange. The dropouts are spaced for 5/16" axles, and you'll be stuck with that unless you decide to file the dropouts. It is a fine frame for a singlespeed, but they were designed to take a three-speed hub without modification.