I do understand, you just haven't answered any of my questions? this will be a concern for anyone that would be interested in quoting on any work for you.
( as free advice you will not get any quotes that hold any water unless your prepared to give detail.)
As this doesn't seem important to you, and you need a flat rate with the understanding the costing will change when the project starts and more features are added id be happy to quote the following to secure the work
My Flat Rate is £0.00 (to secure the project) then with every additional item / new feature / that's not been disclosed above in full detail and or is added to the project will be charged at £2000.
Based on the project info above I'm sure I wont be beaten on price.