Informix 7.0 and SCO OpenDesktop 3

We're in the process of setting up a backup machine for our Informix database, running OnLine 5.0 on SCO Open Desktop 3, and found that ODT3 is no longer being widely sold. We can purchase it, but we can also purchase OpenServer5. Should we instead purchase OpenServer5? Our concern is that Informix 7.0, which we may move up to during the year, may not be fully compatilbe with ODT3. If OS5 is not necessary, we would prefer to stay in ODT3. Please e-mail replies. Thanks in advance. -- If I post often, will Coca-Cola buy my .sig space for advertising?