Hello,
How much content and what type of content do you wish to display on your clients sites?
The way I would approach this is to setup a portal for each of your users, so that takes care of the URL issue.
On your main site, make sure that the content you want to duplicate on the other portals has an RSS feed.
On each users portal I would set up pages that display the RSS feeds from your main site.
This way any updates you make to your main site are then displayed on all of your portals.
This will also give you a lot of flexibility for the future where you can add as much custom content as you require for each of your users.
It may be worth looking at this issue where we cover RSS in detail:
Once you have created 1 client portal, you can create a portal template from the clients portal, so when you get a new client you just have to apply that portal template, this will setup all of the pages with the RSS feed for you, so this will save you a lot of time configuring each portal:
That is how I would approach it, does anyone else have any suggestions?