Hello and welcome.
What is it you plan to do modification wise with the car? A lot of the time people buy a Wrx modify it to a point and regret not getting an sti which is a better base model to start on.
I have had an 2003 Wrx, a 2006 sti and now have a 2004 sti and out of all of them I prefer the 2006 model looks, toys and build quality but the 2004 is a great car for what I am looking for which is really just a weekend toy/track toy.
The 2005 sti models do have better engines than the 2004 models I believe but not sure if the same goes for the Wrx versions.
And Paul has already said give Andy forests website a look and that should give you a rough idea on east should suit you as a goof base for your modification route.
There are a few Wrx for sale on Facebook one being a 55 plate black Wrx with 68000 miles on it at £5400 with some nice modifications for sale near Leighton buzzard(not sure where that is) and has a full service history.
Chris