A call option contract is a fairly standard form. Unless you are having your solicitor re-invent an already created wheel, it should just be a matter of your solicitor's time to explain how it all works to you.
The call option premium can be any figure from $1 (or 10c really) up to whatever you and the vendor can agree on, that you will forfeit to the vendor if you do not proceed to purchase at or before the agreed date & time.