Thing about the KA24DE is the fact that we didn't get any, we got S13 chassis cars with the CA18DET (lots of these about) and S14 chassis with SR20DE's or SR20DET's. Pulsar/Sunny GTiR's aren't very common realy, so most SR20's are gonna come from an S14 more often than not.
Bingo, no KA24's here, so it's either a CA18 or SR20 if I go along that route. As for having one shipped in, no engines here = no need for parts here. Having to have every part I might need shipped in wouldn't be all that fun, even if a <£250 engine sounds tempting.
The 3SGE is a possible choice, there are a couple of 3rd gen motors on ebay at the moment at reasonable prices, so I'll keep that one in mind.
The SR20DET would almost certainly be too expensive sadly, but the CA18DET could be a possibility. It has ample power output and plenty of aftermarket parts, but I'd need to find somewhere to put the intercooler.
I did take a look at a couple of Ford V6's as a slightly obscure choice, possibly a Duratec 25 or 30. I'll have to look into them a little more, but they seem to be pretty solid engines.
The Honda B series engines are possible choices, although I'm having some trouble finding prices for any. I'll keep hunting.

The K-series engines are way out of my price range, so they're out.
AS far as I know we never got any GM Quad-4's over here, so I guess they're out aswell.
Thankyou to everybody who's replied so far, it's been a great help. 👍