I\'m about to begin working on a web page with a complex table-based layout (coded years ago).
One of the things I\'d like to do is convert the layout to a proper CS
The converting process will be a painful headache! I suggest you to start a whole redesign.
I wouldn't use a framework. Learning a new framework is only useful if you use it over & over again. Each framework has its own bugs and weaknesses. Use
display: table-cell;
to make a column. These will line up like float: left;
. See http://quirksmode.org/css/css2/display.html