I had the 45 2.8, the 80 2.8, the 150 3.5 and the 210 4.0 'C' lenses. I would vote for the 45 but that's gonna set you back a bit. Get the 80 and let them get their feet wet and then branch out from the norm on their own. My $.02
Oh, and at http://creativeimagemaker.co.uk I wrote a review on the Sekor lenses for the 645's last year. It might help.