Most CMS solutions will integrate well with your site designs - they don't actually change any of the designs that you have already set up, they merely allow users to easily place content in the areas that you have already created and are allowing the CMS to add content to. So for instance, if you have a page with a content div, and you have already set the styles for that div in a stylesheet, then whatever is put into that div by the CMS will initially take on the styles that you have assigned to that particular content area.
However, there is nothing to stop a client going bonkers with large multi-coloured bold text if he wants it to look that way.
Most CMS systems use a database to store and update any content input by the CMS system, and HTML editor is really just a tool that allows that input to be formatted properly (and images placed etc) but is not in itself a CMS system.
There are free resources (Cushy CMS is one, and quite a good one) but none of which gives you a truly independent 'in-house' system to deploy in as many sites as you like. InContext by Adobe is another rather slick application, but the likelihood is that Adobe are going to start charging a monthy fee for it, so people are very wary of it right now.
Another version is InteractiveTools' CMS Builder, but it costs $199 and it's only for use on ONE site (it won't allow you to install it on two!) - and you don't find that out until after you've bought it.
So if you want a genuine impartial recommendation, I would actually go for WA's PowerCMS - I have it on three sites now (incidentally replacing one Cushy CMS setup because the user couldn't get to grips with it) and it works well and the clients like it. I wouldn't recommend anything I didn't use and like.