I wouldn't buy a new hub unless the rim was also worn out, in which case you might as well buy a complete rear wheel. Too much effort to rebuild the wheel just for a malfunctioning hub.
The SRF3, although it is immune to slipping between normal and high gears, isn't as simple and reliable as an AW, and s/h AW hubs are far easier to find, many of them very lightly used*. No need to change anything else as the SRF3 uses the same shifter, cable, adjuster, sprocket etc. Older AWs were designed to run in oil, with an oiling point in the shell, but you can equally well smear the mechanism with SA internal hub grease (the brown runny stuff) and forget about it.
*a worn one is fine if you change the clutch and planet pinion pins for about £5.