I agree on a used Tach or Zone VI and a couple of lenses, maybe a 210 and a 90. It won't set you back all that much.
We've been shooting pro for 20 years, picking up digital about 10 years ago. Throughout that span there has been a constant, low level need for 4x5, to the tune of roughly 200 frames a year. All but a few uses centered around the need for the 4x5 body movements. In the last few years 4x5 is the only film we have shot, but I think once you become accustomed to the gear and understand it's specific advantages, you to may find occasional uses in your pro shooting.
For the learning, I'd recommend also picking up a Polaroid back, which you can use not only to pop a few polaroids during the early learning phases rather than waiting for film processing, but also as a general purpose film holder for quick load film. We never use our mountain of film holders for sheet film any more, instead using quick loads for convenience. That's a good approach for learning due to ease and convenience, even if more expensive that sheet film. If you're shooting lots, yeah, move on to sheet film. But the savings will come at the expense of effort in the short term while you're learning.