Seeking prevention for POS systems, best practice

I'm taking on a POS business, and already the owner of the business is discovering clients that are easy pickings for today's scary new varieties of malware. And of course once infected, the security risks are huge and so is the labor involved in eradicating an infected POS network.

I want to start a practice on these machines that is the best hope for protection while at a minimum of cost and labor.

The machines typically run XP professional or XP Embedded, plus one of the more popular POS software applications, and typically use a standard router like a linksys. Typically one finds 2 POS machines as restaurant or retail terminals, and possibly a third for management in the back.

So far the owners' current plan is: Install malware bytes premium, install windows defender, install AVG, and apply all updates. Is this adequate? I have my doubts.

Could I get some pros here to recommend the best setup that is neither high in cost or labor intensive?