When I say "Custom" leftie deployment is all I was looking for, and emergency procedures are no problem. However, I've seen recommendations that a used rig may have a much less comfortable fit than one that's custom built specifically for me. If there would be a very noticeable difference, I'll always take quality over price.
When you say 'custom' what else do you need besides a left hand main deployment? If everything else is the same, all you need is a BOC pouch sewn on facing the other way, not really a big deal, and not all that 'custom'. If that's all you need, you don't need a new container to have a left hand deployment, you can just have a rigger sew a fresh BOC on facing the left side. Truth is, a fresh BOC when you buy a used rig isn't a bad idea at all, no matter what side you need it on.
Are you good with your emergency procedures? Can you pull the cutaway handle with no problem, and with a good deal of strength?