Buying a modern hubset is not out of the question either, and I gather the newer Sun Race/Sturmey product are very high quality.
well, from a mechanic who works in a bike shop in Paris, I am afraid to say he mentionned modern sturmey were not at all what it used to be so I would be wary of them.
I actually read also that in the nineties, quality of Sturmey went down too. as the company got into financial trouble.
According to this mechanic, the 3 gear ones buit until beginning of eighties if in good condition are not indestructible but close.
5 speed one are not widespread at all, were mounted on moulton but apparently discontinued so not a very good sign.
Nexus / Shimano would probably a better choice today, unfortunately, they surely do not look OK on a retroframe