Hi,
I have a lot of experience in linux and wifi networks and also with multiple vendor captive portals.
I have do this project as you require to your specifications if you would consider a currently solution already created but tailored to your request?
WiFi Dog is a solution that runs on OpenWRT and I can customize this to your requirements to save you time and budget as to write something like this from scratch the budget would need to be a lot higher for a good stable product.
Thanks,
Andrew