i don't know if there are any software suites designed for this specifically, but you could certainly arrive at that end with some work. try this... http://www.linux.com/base/ldp/howto/...WTO/setup.html
a pretty neat and very specific howto i didn't know existed. would look to do everything you're after in that angle. I assume that the existance of a user is based around some sort of domain membership and authentication that way, so you'll be needing to (presumably) plumb the depths of samba to actually get that "transparency" but the howto above gives examples based on explicit authentication via a portal and such, which may suit your needs.
you may also find that other appliance solutions, like Astaro and IPcop may cater for this sort of over all solution in a much simpler fashion. Astaro is a proprietary system, but free for home use and they're generally very good guys, so might be a justifiable cost.
note that there is a *huge* danger trying to transfer from a M$ solution to an open source one as often the costs involved are assumed to be nil. just because things use open source and such doesn't mean there's a drop in, low maintenance solution to suit everyone's needs. people often end up thinking "this linux stuff is shit compared to M$" when it's not trying to compete at the $0 level... Hence why companies like Astaro are providing a commercial yet open source derived solution to try to equate ISA.