I would like to be able to vary the fan speed and drum speed of the roaster.
I've never used a a VFD before. Can I just install a VFD like this for each motor and hook them up to Artisan through the RS485 I will be running for the PIDs?
You cannot, as those drives output 3 phase AC.
You can use an SSVR (solid state voltage regulator) as some of the other roasters (North) do on their blowers with some success, but the drum is usually better handled by a DC motor and controller (cheaper but requires mounting a "different" motor footprint) or by switching to a 3 phase motor and VFD drive (more costly, but you can mount a direct-replacement for your motor).
You can try the SSVR approach on the drum, as well, but it's a less common solution and you'll run into problems if your motor uses a start capacitor with an inertia switch, as they dont like running below the the minimum speed that opens the start capacitor circuit. Fans are generally equipped with a start winding and a permanent capacitor, so they fare better with SSVR speed regulation. SSVRs can be had for relatively cheap. But they generate heat and need to be mounted accordingly (heat sink, good circulation, etc...) lest they burn up.