Hi I'd suggest getting there early. Doors open at 11am but the queue will start soon after 9. There are definite bargains to be had and only so many on offer. However it doesnt sell out til after 2pm when some ppl may start shutting up, though most ppl will be selling up til 4pm. I suggest you take a look at thespidershop to get an idea on prices and what you're after then start looking for prices below that.
I don't hold much hope for P. metallica, it seems there are some greedy ppl out there.
I'll have a table selling and advertising Arachnid Addicts, so come over and say hello. The whole event is very friendly and a great way to meet ppl from the forums.