Classic mode can use a lot of resources. So it may slow your system down a little.

As far as helping goes ... some of us spend many, many hours online. (We're all volunteers! Not enough pay for us!)

I have 2 internal HDD's which are all partitioned. I save my downlds on a different partition to the OSX system drive. If you have a 2nd drive then that might be a good move. Otherwise just be careful. BTW you can check how much RAM & VM LW is using by opening the Activity Monitor which is found in the Utilities folder.

As for port forwarding. The instructions I have are for windows so I'd need to improvise a little with OSX instructions for setting up a static ip which is required for port forwarding. But it would still be extra helpful if you could find out the model number of the device. I presume it's listed in the 1st section of the info about device. Click on the + sign to open up more options.
Also look for UPnP in the routing section.