04 DE is worth about £6K unless you find someone special.
Even if we assume you get £7K for yours, another £7.5K gets you to where you want to be. By the time you've bought the Evo at £13K you're talking a cost of £6K, and that assumes there's nothing wrong with it or that you don't decide to mod that too (yeah, right!). You're left with an unknown entity of a car that could be perfect or could be mullered, Is that worth it for the saving of £1.5K? You'll spend that on the extra fuel and servicing of an Evo in the first year.
By all means get an Evo if you want an Evo, but don't kid yourself with the man maths.