Welcome to the world of PD150 ownership! I've put 40K on mine in the last 15 months and have had great fun doing so!
I am toying with the idea of Sat Nav in mine, and if I do go for it I will be installing Kenwood stuff. Its not the cheapest out there, but arguably one of the best systems available today.
the sat nav unit itself is DVD based and contains all of Europe on one disc. It works off both address and postcode recognition. Has numerous cool features such as POI (Point of interest) which includes petrol stations, hotels, garages, etc etc.
Match this to one of their KVC 920 headunits (as a replacement to your clarion headunit and changer) and you'll have a fold out, 6.5" touch screen, which will not only command the nav, but add TV and DVD to the list.
All for around ?2-2.5k. Not cheap, but about as cool as it gets, with the added bonus of TV and DVD.