You have a very simple (and great), static website that does not seem to open up a lot of programmable interfaces, other than the forum.
Assuming that you update all your software periodically, you should not be at much risk from anything other than the forum software.
As for being probed, all websites are being probed continuously by automated scripts.
It is my opinion that the biggest risk to your site is your forum software. Personally, I do not trust any PHP based forum software. If it was my website, I would personally host the forum with some security separation from the main website, such as on another server.
phpBB3 has had no major known exploits since it's release in late'07, when I say major I meant things like SQL injection. Secunia lists 5 exploits found all of which are low level except one. The moderate one was a bug when they introduced RSS feeds that would have allowed someone to read private forums when some other conditions were met....... unlikely these other conditions would be met on most forums anyway and that was patched in days.