There's a good thread here:-
http://www.peachparts.com/shopforum/showthread.php?t=253687
Although it mentions brake lines - fuel lines are also talked about.
A summary is that for fuel lines the easiest way is to run a replacement plastic line along side the existing lines - tie it as you go!
I've recently replaced all of the hard lines (brake and fuel) on my W123 and I bought almost all of them from the dealer. The return fuel line cost me about 70 euros and the long brake line was about 50-60 euros or so. The dealer hard lines do not come pre-bent so if you can find another way of doing it cheaper then there is little advantage using "the real thing". For the pressure fuel line I ended up getting copper as aluminium or steel was unobtainable over here. The downside to copper is that if you use or plan to use WVO or Bio diesel I understand a horrible chemical reaction occurs...
To replace the hard lines I think the easiest way is to remove the sub frame on a W123.