Why would any buyer want to sign a commitment before you do any work for them?
What happens if they sign the commitment and then they find out you are lazy and are not providing much help?
That's what a buyer agency contract is all about. You say you will do 1...2...3...4... etc. and if you don't meet the buyers expectations, the buyer owes nothing and can look for another agent.
However, a retainer fee to cover expenses might be appropriate. A short trial period, to measure being able to meet each others expectations, might also be appropriate.
Work, what's best for you, your budget and your business plan.