Search the forum as this topic comes up regularly. Cutting to the chase, the 645 is a very old model, approaching 20 years old I believe. We don't have an SCC but did look at them a couple of years ago and concluded the 618 is the winner, since it is fast and the tools are removable. So if we ever get one, it would be the 618.
Buying a 15 year old SCC is akin to buying a 10 year old digital press. Generally it's been ditched for something more modern because it's become tired/expensive to maintain/inconsistent in output quality/reliability/no longer commercially viable. Personally I wouldn't touch it, as it's likely to become a bottomless money pit and even then, there's no guarantees it will ever be a reliable asset that will make you money, instead likely to introduce wastage and reprint costs. Buy once, cry once, and get a 618 (if you really need an SCC, that is), not a boat anchor.
