I've had loads of experience with actinic, which version is your client planning on using?
it doesn't have any killer features when put next to any half decent OScommerce store (my experience is of versions 4 - 7). People like because it's supposedly very easy to set up your store and get trading (but probably no easier than any half decent os commerce blah blah).
the downsides of actinic is seemingly endless variations on their very bland templates, if one wants an actinic store that looks nice, they'll need to find somebody who specialises in custom templates for actinic, which adds to the cost of running the thing.
there are plenty of plugins available for it from
www.mole-end.biz and whatever site a bloke called Norman Rouxel has.
The Actinic community is pretty good for tips and trouble shooting, but they're slow to respond (especially the older versions).
did you say a £400 price tag? the latest version 8 is £399 + vat and you could get them down to £350 if you ask nicely or go to a reseller.