Rental contracts are a bit more important for us visitors residing in thailand.
They form part of the procedure when you require certain types of Visas.
For example.
Not sure if all immigration offices insist on this but some require the contract + the owners ID card and sometimes even a photocopy of the page of their green/blue? book, which is a government issued ownership document. The immigration also require those documents/copies to be signed/dated by the owner.
It would be wise to make sure your realtor is onboard and aware of these requirements.
500 baht seems reasonable. Those fees are common, i have known those fees to be as a high as 3000 and as low as 160 baht per month during my time here.
Your water and electric bills will be in the name of the house owner. Easily payable by visiting a 711 and presenting them with bill, pay and get the reciept stapled to the bill.
A lot of agents push for 2 months deposit + 1 month rent in advance. I have never paid this but i have also never posed an issue until i am with them face to face. I did get my full deposit back from an agent, be prepared for agent to deduct cleaning fees from a deposit.