Hi mate, Calders are like every other dealer some people will have good things to say others bad. I bought my V3 STI 555 from them last year and as double zero says it's mint. Low miles and one owner in Japan with full history. I cannot fault them for their service, got my car with a warranty, damage to the front bumper repaired,new clutch and full underseal for nowt extra. I cannot recomend them highly enough, if you go ask for Dee.
I also notice that all those with bad things to say have only been up for a look and not actually bought from there, yeah the cars might look a bit tatty if they've not been valeted yet, bear in mind they spend several weeks in a container to get here. I personaly think it's better seeing them before they are valeted, that way you are seeing them warts an all. A guy local to me has just bought a white V3 STI from them and is delighted with it.
Honestly mate go for a look it'll be worth it, and as for being overpriced have a look at McKay Trading's website and you'll see overpriced.