With the economy in the dumps, you may be able to find a used system that is current to this decade. Look for the Strata CIX in the Toshiba line.
The next step above a CIX 40 would be a Toshiba CIX 100, which will easily accomodate more the 25 digital extensions. The latest CIX 100 processor is the ACTU3A, which is more recent then the 1A or the 2A. The latest model of phones are the DP5000 series, in case you were looking for this stuff on ebay or something.