Hi good luck with your choice, buy the best you can afford is my motto... re LE, longest i have shot with the 645D is 4 mins, copes with it very well, make sure you also invest in a SOLID tripod as well though and use the little IR remote ..all basic stuff which i doubt i need to mention... not into star trails etc per se, really only use the LE for smoothing water and cloud streaks... re the bloody dark exposure i, i think its down to the heat generated by the bigger sensor... there are threads that say it can be turned off, but i have tried and updated firmware and still it cant be turned off, pleasde correct me if im wrong.. So it comes down to timing on sunsets/sunrises... as time and colour moves quick at these times... good luck... Go the Phase i guess if you have the cash..
I also print large have a Canon IPF 44 Inch and the prints off the Penty are fantastic...