Starting my own painting business & having trouble with pricing things!

So, I am trying to start my own painting business & I really don’t have any idea how to go about pricing things. My first client wants a nursery painted & the couple wants me to just design something really cool. (Neither care what it is). They told me to just let them know about pricing and everything.. how do I determine this? I did just recently ask them if there was a budget that they are trying to stay within. I haven’t heard a response yet but I felt that was a good question I guess. Also, should the price be based on these factors: how detailed the design is, how large the room is etc.? Should I do an hourly wage, or have a flat rate? Should I have them pay in full, or have them give a deposit/half initially? Should I create a contract, in case they don’t pay? I realize this is entirely up to me but I am oblivious to the mannerisms/management side involved in small business. I would like to add… which is a major factor in deciding these things is that I used to clean this guys house every week. The last week that I cleaned for him he did not pay me, idk if he forgot but I told him & was never paid. He is a very busy guy but I do not want to get screwed over.