Porting DOS Applications to Unix?

jgbustos asks: "I'm the manager of a small software development company. For the last five years we've been managing a communications network consisting of several thousand PC terminals which connects to an NT server over the phone lines. The terminals run a console application on a light DOS. Everything is fine so far. So what's the problem, then? We have a hard time finding parts to maintain the network. Our client says that he's willing to invest in hardware upgrades, but we should move to a Windows-like environment, however he doesn't want to pay for more than 4000 Windows licenses. What's the best alternative for porting our app to Linux/? Please bear in mind that the terminals are small PC's having 16MB RAM, less than 2GB HD, less than i586 CPU, and a 33k modem. Thanks in advance"