Since you have to upgrade "major" items i would sell on.
I own a Lexus IS200 (used to be a 2005 Leon 1.9TDI Stylance) and the forum where i come from, people are swapping the IS200 engine for 2L turbo engines, but there spending 6K on doing them.
If your up for spending the money, do it. You won't get your money back.
I know someone who bought an IS200 for £6500, has spent 10K on an engine swap & other bits, and is now selling for £5500!!