Stuart, the best lenses to go for are :-
RODENSTOCK RODAGON
SCHNEIDER COMPONON S
NIKON EL NIKKOR
These are all 6 elements in 4 groups
I have bought lots of stuff from -
http://www.secondhanddarkroom.co.uk
I am not sure if the 80mm lenses have enough coverage to do an X-Pan but the 90mm (6x7) or 105mm (6x9) will cover any of your needs.
However, if you have a 504 - you might want to go to a 150mm lens and then be able to cover everything upto 5x4 - it just depends on how much money you do or don't have and what your ambitions are
Nothing is worse if your on a realy tight budget than having to buy additional stuff because you didn't quite think it through enough before you bought the first time
The good news is realy good englarger lenses are cheap second hand - everyone seems to be selling up their darkroom kit
Have fun & good luck
Martin