As a developer, do I need Mountain Lion?
Mountain Lion versus Lion is one thing, but it's worth considering that, with Apple's apparent intention of releasing a new major version of OS X every year, a machine that can only run Lion will rapidly fall far behind the baseline. This may make it hard to develop against new SDKs that require the latest version of Xcode.

Buying an older Mac mini may mean saving money now only to have to spend it sooner than you'd like to remain productive.