Can I edit the wording in the invoice link inviting clients to pay online? I want it to say that they can pay via PayPal.

The text for the link is fixed text. You can however "Add A Note" (and make it a "note template" that you can use each month). When you add a note, that message is included in the body of the invoice, just above the link for the client to click on and make their payment. When you "Add A Note" you could say: "You may pay this invoice via credit card or Paypal account at (link provided below)".