and have noticed they have fans installed in them.

My original TM-NANO that I bought last year doesn't have a fan installed and only has one usb port on the back whereas the new ones have two.

I'm presuming that this is a new revision of the box but it's made me wonder about whether I would still need to purchase a HD caddy kit if I wanted to install a hdd.

From my understanding the fan was only required if you had a hdd installed.