I may need to withdraw more Baht when I get to the ticket counter, and so I am wondering, are there ATM machines within a short walking distance of the ticket desks? If not, where are the nearest ones?

on the Mezzanine level there are several (that is where all those Restaurants and Coffee Shops are).

As well in the departure hall on the right end (if you stand in front of the terminal-building), there is a SCB branch with ATMs too

Or if you walk through departures (past check-in desks) towards security, there are several exchange booths there at the back of the departure hall which have ATM's.