I think if I had to do it again, I'd replace the entire rad assembly with an all aluminum setup. They're not really that much more are they?
Otherwise, its a case of you gotta pay to play. Not much out there competes for the price point of a 540i right now.
Perhaps in the next few years the some of the big motor Mercs might be a good deal for a suitable replacement for the V8 e39, but I bet they'll be just as bad for repairs.