Andy not good news really, the off set of the skyline rules out most of the European spec after market alloys in 18 and 19 inch, You have to use Jap spec produced really and these don't come cheap at all!
Stock 17s off 33's are fetching £500/£600 with good rubber still and I know of a lot of people holding cash looking for the same as yourself.
I also know (as I bought some last year) that add the import taxes etc and some good rubber you would be talking £3,000 + for Nismo, BBS, Volk Racing and Enkei which are the most popular in Japan. Thus making those wheels on a second hand market and in good nick at least £2,000 with rubber.
Just my 5 pence worth thats all.
...Ian (Nismo GT3s 18x10 Supplied and fitted by Abbey MS)