I completely disagree with all Mac Mini suggestions.
Get a macbook. Used or new, depends on what you prefer. Or better yet - get the Air which is same price than the macbook.
Today a new mini is U$300 cheaper than both the notebooks (which goes for U$999) but you don't have to deal with keyboards, mouses and above all lack of battery. Any computer without a nobreak is a break dealer for developing, from my point of view. Adding all that plus the space used, power consumption and convenience we can easily sum up to macbook being a lot more worth it. Plus, with the air, the SSD makes it worth even a lot more.
The mini is good as a home server or any kind of dedicated machine. Never as a desktop nor to have frequent user input - even if it can be set up for such just like any messy PC.
Anyway, I'm a long time PC user, I never dropped windows or PC, but if you're going to develop to mac, please, go all in. Learn to enjoy the apple way, to hate it as well, and make that software really a Mac OS one, not a ported piece that doesn't fit in. There are many, many nuances to a true software made for mac that can make the difference for your software being successful on the apple market.
You can even use bootcamp to install windows on it if you wish, but like everyone else said, as a developer you are going to fall in love for the mac OS and the amazing hardware (which won't happen if you go Mini).