Developer who is familiar with Paypal checkout

Need a dev to advise on building the checkout buttons for my website, I'd prefer to use the pre-built butons for check out however I'm still looking to see how to include some custom information such as user details (for digital goods).
Then on the receiving end either use IPN or similar to process that.
You will need to talk me through what are the options, further work could be optional.
Start your bid with "I do paypal"