Hello & welcome,
as for the impreza I'm sure one of our owners will be along to help out, have a look in the buying guide for things to look out for,
Main differences between an 04 and an 06 car will be the running costs, more specifically 06 cars fall into the new high level tax bracket 450+ where as the 04 car will be 280ish I think, general running won't be much different, as for the two examples you've picked the 1st one would need a friendly mot tester to get it through each year due to having no cat. In some cases having modifications will bring the insurance down as the company sees that your an enthusiast and takes care and has put money into it and not a complete numpty, if you did decide to get one thats has had some stuff done ask to see receipts just dont take somones word for it that it has millions of horespower,