I'm in the same boat as you, 2004 GT tdi 140 losing water and siren turbo !
Mind you, changed the EGR cooler last weekend and it hasn't used a drop of water this week so fingers crossed ! Just got to sort the turbo.
I also thought of changing mine but the cars out there just don't compare to this one, i was thinking of a 1.4 TSI but less MPG, more road tax and having to shell out £3000+ for a cheap one changed my mind. Better the devil you know I supose so I guess it's new turbo time.....
Trade in value for mine on a £7k 1.4 TSI was around £3000-£3300 (FSH, 115000 miles standard spec). There's loads of GT's out there now from £4k at dealers with FSH and warranties !
Keep it and sort it !