I should also make it clear that I don't want you to share your financial situation on this forum. I'd definitely want my teenage son to be very careful about what sort of information he was posting about himself.

To get an idea of what kind of bikes other people have used for what you have in mind, go to google images and search for "surfboard on a bike". You'll see some expensive fat tire bikes and some cheap cruisers. All would probably get the job done for you. Fatter tires with some tread are going to work better in the sand but will slow you down a bit on the street.