I never used Quickbooks before. Do I need a merchant account to accept payment when I invoice a client? Or is linking my bank account enough? Thanks!

To accept a credit card or ACH payment via the QBO customer invoice you would need to setup "Payments". But you can still give your ACH banking information to your customer to make a payment for an invoice and expect that payment to show up in your connected bank account feed. In this case you would not be processing the payment via QBO Payments (merchant processing), this would be a bank-to-bank ACH transaction.  For credit cards payments you would need a POS system (PayPal/Stripe/Square etc.) or a virtual terminal of some kind. If you setup QuickBooks Payments the ACH type payments are free and your customer could pay via the electronic invoice. There is a fee for credit card payments.