A modern ResMed AirSense 10 AutoSet has an "EPR" setting that can reduce exhale pressure by up to 3 cm-h20. Poor man's BiLevel/VPAP. Start with that. If it's proven you need more than 3 cm-h20 exhale pressure reduction, worry about it then. I imagine there is an equivalent Respironics machine.
Depending on how your insurance works with co-pays and deductibles, it might be cheaper to buy the machine outright online. Paying cash, you can save a few bucks by buying the last generation ResMed S9 AutoSet. It has identical functionality to the latest A10 other than the built-in modem. You can also buy an S9 AutoSet used with a 1-year warranty and save even more. An A10 with ClimateLine hose will be about $900 new on the internet. A used S9 AutoSet with humidifier and ClimateLine heated hose will be about half that.