If you get a super deal on a 30, it's not a big deal to convert a 30 to a 25 - much harder to go the other way. For some models, all you need a 25 driveshaft, shift rod, and water tube. Take the 5" spacer out from the mid, change the shaft, shift rod, and water tube, and put it back together. Now you've got a 25. Sometimes you can cut the water tube and not have to replace it. And lots of shops have driveshafts laying around...
Good luck!