Assuming periodic breathing and central apnea are not issues, consider first whether or not your greater need is for ST mode (forced breaths) or VPAP functionality (varying IPAP and EPAP) or AVAPS/iVAPS (keeping a certain tidal volume going). Apart from lab machines, which are sometimes available, no machine does all these things equally well and none operates on more than one mode at a time. If you need pressure that rises and drops based on apneas/hypopneas or tidal volume throughout the night/day (a la VPAP S or a VAPS mode), you should also consider the extent to which you need EPAP to vary independently of IPAP.
Then go on SecondWind or somewhere and buy gently used (I prefer ResMed to Respironics but YMMV) so you don't pay too much, given that you may need another option later in your disease.
And to all, spring for the data card and download SleepyHead if available for your machine so you can track sleep daily and export data easily to your clinic. The proprietary Respironics software is much more cumbersome, one of the reasons why I would try to buy within the supported System One or S9 series (both supported by SleepyHead, which is cross-platform) if possible.