I reckon it'll go for around $1000, anything over that might not be worth it. Bit hard to say going by the photos and limited info. But the rego is alive and that's important.
Might of been an 1800 you were in, would depend when he had it etc, most of the ones I've seen for sale here have been Princess 2's, which it seems most of them sold here was 2L O series, I don't think they sold the 1.7L here (no tax advantage etc like the UK I think).
And they were made here in NZ from knock-own kits, in Nelson, if I recall correctly.
Also, my future father-in-law knows of a local man (he's in Invercargil) who has over 20 Princesses!
Some is rich, some is poor
That's the way the world is
But I don't believe in lying back
Saying how bad your luck is
Awesome! Over 20 you say.. always nice cars that pop up in the South Island, yes I recollect they were assembled in Nelson and sort of filled the breach after local assembly of Triumphs finished... I had a few Triumphs locally assembled Toledo, Dolomite (english assembled) 2000 and 2500 Mk2 and 2.5 PI my favourite after the Dolly.
Favourite memory - touring the Todd plant in Lower Hutt with my parents aged about 10 before they took delivery of a new locally assembled 68 Valiant.