Engine change.

I've owned my Tithonus for 8 months now and I've had a fair bit of work done on the original 2.5 n/a, it's in good health with a recon pump, head gasket and cam belt all done, however I'm tempted to have a 300tdi conversion done and power steering fitted and wondered if this will affect long term value? The 2.5 is OK for the green laning I do but it's getting to and from the green lanes that's the pain as it's pretty bad on the motorways with a top speed of 57mph, that's on the flat, add a long climb and it's more like 45mph.
I was looking at the Glencoyne engineering conversion, does anyone have experience of them?

I did a 300tdi conversion myself and found information from the Glencoyne Engineering's web-site invaluable. He does know his tdi conversions.
When I did my conversion I also fitted power steering and a higher ratio transfer box and it totally transformed it. With the N/A diesel I just about kept up with trucks on the motorway and really struggled on hills, now with the tdi I can overtake trucks, going up hill with a loaded horse box on the back. It will do 80mph+ without a trailer on.