At the moment I am studying for several certifications, this involves
plowing through many adobe acrobat files.

I don't want to purchase the full version of acrobat, but I have a
requirement to set bookmarks, add notes and perhaps even update the
content. What kind of tools are around that will allow me to do this,
I don't really want to spend more than $75.

Here's one for about $65.00

http://ctdeveloping.com/ctdeveloping/products/pdftypewriter_pricing.asp

Since most PDF tools are very expensive, you might want to also
consider using a converter.  Check out either of:

http://ctdeveloping.com/ctdeveloping/news/news.asp?event=9

http://www.surefiresoftware.com/midas/main.php