I'm not sure exactly what type of software development you'll be doing, but I'm guessing it's something that requires a Mac as you already have a machine running Linux and Windows. In that case, a Mini will be more than sufficient for development using Xcode. Even the basic 2.5Ghz dual-core i5 has a fast enough CPU, but if you have enough money I'd go for one of the i7 models - they're quad core and that could make a bit of a difference for big compiles, etc. One thing to be aware of - it's not possible to upgrade the CPU, so buy the fastest you can afford. RAM and storage you can upgrade later on, when you have money.
Two recommendations I'd make
- Make sure you have enough RAM. It's probably cheaper to buy this from a third-party rather than Apple and I'd recommend at least 8GB, 16GB if you can afford it. It's easy to install yourself (just remove the bottom cover and slot it in) and won't void your warranty.
- Get an SSD. Again, it's cheaper to go third-party than buying from Apple. Installation is a bit more complex than installing the RAM but it's still straightforward if you're comfortable taking a computer apart (see the various guides from iFixit). Doing it yourself will, in theory, void your warranty, so be careful!
I think you'll be just fine using a Mac Mini for software development - I'm perfectly happy using my i7 2012 model for development.