What to use for development?

I'm a newbie, sorry.
I have a phpBB3 forum which is running successfully. I also have on the same site a GoogleMaps based web page - just a simple link to switch back and forth between the two.
I now want to build a more diverse site with a bunch of links, access to each of the two existing sites/pages, and also maybe to a couple of chatrooms and a business directory database.
I don't know whether to try to use:

a CMS - it seems it may be very tricky to wrap something around the existing sites;
a Template - don't know whether I can just put links in a template to each individual site/page.application? How easy is it to change the "look" of a template?
just try to code it myself in CSS/HTML


Any discussion/advice would be very helpful.

I'm not too familiar with the current PHPBB state but I'd be surprised if you couldn't use it to run a business directory database. Live chat applications are a bit harder but again, there may be some extendable functionality in PHPBB that you can use. I think you'll find it hard to implement a CMS that embraces your current setup plus the extra features, unless you're going to rip everything out and start again with something like Joomla or Magento (to pick two fairly heavy-duty CMS frameworks at random). Building a business directory yourself isn't too hard, with some knowledge of SQL and a server-side language, but a live chat application can be technically challenging and I'd be surprised if it wasn't more cost-effective for you to use an existing solution.

How about using a portal? PHPNuke for instance.
Also I could be wrong but isn't templates just html and css anyway? Learn those two and you're all systems go :D

I'm not a fan of phpBB so I could be wrong on the phpNuke. I know it's something like that.