Make sure the commission agreement is witnessed by a lawyer.
Brokers can be weird at times. After the sale they can convince you that you had agreed on a higher amount.
In fact, include the bank account to deposit/transfer the money to in the agreement. Don't pay in cash. They'll be no evidence you paid.
At times I broker plots but I know most brokers don't like some formality when it comes to making payment.
When receiving payment, I receive in any form but when paying out, either Mpesa for low amounts or bank transfer. I only take cash when just sitting on a fence and people come to make money-noise near me....as in, when am not a key player. Hiyo mimi hukula and fly away.
Anyway, to answer your question 2.5%-3% is good enough for a property worth 27M.