Mk1 or mk2 can be modified to add spdif, but we're talking about needing to make a small PCB, etc. It's not a simple hack.
The mk1 is less suitable for large drive upgrades as it has less ram, and so holding a big database can cause problems with caching. Remember the biggest mk1 ever sold was 20G ISTR. The audio output suffers from more noise problems and the built-in FM tuner isn't particularly sensitive.
Carplayer.com will ship to the UK, too. You could always try mailing
david@empeg.com and seeing if there are any rios left after the current ones go out - there may be ones whose cards were declined (or whatever) which are now free.
The phatbox is a different kettle of fish; it allows you to do a stealth install, but is less satisfying for the music junkie who likes to organise their collection (ie, exactly who the empeg was aimed at).
Hugo