Paypal is great to get started but my experience is that you'll take a hit to your cashflow at some stage. They'll freeze your account, chew up a lot of your time and make you angry. That's part of the cost of doing business with Paypal, so if you need cashflow, accountability and legal protections then go with a bank.
In the early stages, don't be afraid to do a chunk of your billing work manually (producing invoices, providing refunds, calculating pro-rated charges etc). This will give you important insight into customer pain-points and you'll learn a lot about online payments too.