Payment Processing: What should I understand before becoming a merchant of record?

Your payment processor should take care of those, the PCI compliance, fraud management, etc. As an MOR you are simply the beneficiary of the funds through the sales proceeds and bear the end responsibility of the transactional risk, as seen by your issuing bank. You need not go about reinventing the wheel with regards to PCI, etc. Simply choose and work with a payment processor that provides all this services. Customer support is something you would be responsible for.