What paypal system are you using? it would sound like you are using paypal standard.
what you propose has a number of flaws:
what is to stop a person from simply going to the checkout success page and triggering the update?
with paypal standard you would set up an IPN page to do the update. when the transaction is processed, paypal will make an ipn post to a [page you set up, on the IPN page, you can do the update. see here for more details on IPN:
instead of using paypal standard and IPN, I would suggest using Express Checkout, it is a real time payment method, it is also free like Paypal Express and gives the same checkout experience.
Unlike payments standard, the order insert can happen on successful transaction so there is no need for an update.