I have a 2007 SE with the MM pack and to answer your points:
1.is it definately worth going for one with the multimedia pack in?, how good is the sat nav?
Absolutely no brainer. The sound system is brilliant and the use of the rear camera (especially in winter) is very useful.
The Sat Nav is OK but I have found issues in the area I live with it just no recognising certain motorway junctions. Another local owner of an RX said his was the same.
2. i'm struggling to see what the differences are between the models, is the SE-L the top one?
Can't really help but since I did not like shiny wooden inlays I opted out of the SE-L
3. If i end up going for one with say 50k miles on, will this be an issue?, what is considered high mileage on these?
Do your normal checks and I guess the alloys will be very poor at that mileage. Get them swapped. Water pumps go at around 60-70K so listen for a rumble from the front of the engine around the pulley area.
4.I definately want an auto, what are people getting in terms of mpg?
Auto is again a no brainer. Test drive both back to back and you'll be converted for sure. MPG I think I have averaged just over 31mpg over the 40K I have driven and on runs I have had a tank average of over 40mpg.
Hope that helps and buy one. You will enjoy it for sure. ;-)