If you tend to have your clients popping over to the Roastery it is always nice to be able to make them a cup of coffee, not only for testing the product but also for general drinking purposes. Investing in a small quality espresso machine is always worth it as most clients use their coffee for espresso-based beverages. If this is not necessary for your setup then maybe invest in a few manual brewing methods.
Personally, having three types of manual brewing is best. A pour-over style, full immersion and pressure style.
Filter machine, Chemex, V60 or Kalita wave
Plunger, Clever dripper or percolator
Aeropress or Mocca Pot
These methods might not be exactly what your client has but would simulate what your client is using and give you a similar experience to their expectations.