Many commanders complain about Marco Gwent because he is time consuming to unlock and does not go up to grade 5 in anything. But he is more centrally located than the engineer that can do grade 5 power plant engineering, and you rarely need to grade 5 engineer any power plant. If you are doing overcharging, only do it to the grade you need to provide the power required, as overcharging increases heat output.I was planning to got to the engineer in the Sirius area next, as he is advertised as working with power plants.
To access Marco Gwent you need to be allied with the Sirius Corporation (run missions for rep+ with that faction), find an accept the permit unlock mission to get access to Sirius and then find 25 Modular Terminals. Sometimes you will find Modular Terminals for sale on fleet carriers. When I get some I put them up for sale on my carrier at the maximum price I can set and they are gone inside 48 hours.
Modular Terminals are only available as mission rewards and are more common in high tech economy systems. So you need a ship capable of running missions where passenger missions also work, and have 25t spare cargo space, and evade pirates after your cargo the whole time. I can take 2 to 3 hours of running missions to collect them all unless you get lucky.
If you only need grade 1 power plant engineering Felicity Farseer is much easier to access. I've used a Python or Krait II to run around and unlock Marco Gwent the two times I've done it. I needed a ship that big to fit a passenger cabin or two plus cargo space. You might be able to make it work with the Dropship.