That sounds right! I bet you can use the hardware you got with Rocksmith.
Scuffham comes with a standalone player, but you’d have the same problem of figuring out how to play along with a CD or MP3. It has a free 15-day trial, so give that a shot to see if you like how it sounds and the features it offers. You can also use it to see if most software can use your Rocksmith audio interface.
Just install it and run the standalone, and click File -> Audio Driver Configuration. See if you can find the Rocksmith interface in the list for “Driver:”. If so, pick it, and see if it works.
If not, and if your computer has line inputs, you can try your computer’s built-ins, but you’ll need to install ASIO4ALL driver to make your built-in drivers low-latency. (Without ASIO4ALL, you’d hear what you play almost a half second after playing it, which is no good.) It’s free, google to find it. I have a MOTU audio interface, but I often use ASIO4ALL with the built-in just for convenience. You’d also need an adaptor cable. Ideally you’d want an instrument preamp, but since we’re on the cheap here, you can plug an electric guitar straight into line inputs and they’ll work. (If your guitar has active pickups, there’s definitely no need for a preamp.)
Unfortunately the S-Gear standalone doesn’t have a built-in media player.
Cantabile allows you to load “media files” and play them, but it only supports .wav and MIDI files, so you’d have to convert MP3’s to waves, which is inconvenient. So the best bet there might be to find a free or cheap “media player vst plugin” and use that along with whatever host you select.
BTW, there’s a free VST host, which I used for many years. It’s fiddly to set up; it was intended to be a host for testing VST plugins. But it works, especially if you like having one setup that you don’t change much. It’s “VSTHost” by Hermann Seib. Definitely worth trying out and saving you $100 for a host like Cantabile or Reaper.
For example, here’s a very simple free one: http://www.vst4free.com/free_vst.php?plugin=vstPlayer&id=972 (didn’t try it)
I’d like to find one that supports pitch shift, so I can learn tunes in the key the vocalist wants. I had found one years ago, but for some reason it didn’t work out well (don’t remember why, but it was a convenience issue.) I think it was this one: Askywhale's VST (scroll down to Mp3play VST).

