I think it's a good deal. You can always wait for a new Mini to arrive, but there is no guarantee of when (or if) such a device will arrive, or whether it will be a significant upgrade either (the 2014 was not much of an improvement on the 2012 after all).
It's easy to use an external SSD if you want better long-term-storage performance. The only thing to note, of course, is that the RAM cannot be upgraded, so be sure that you'll never need more than the 8GB that it comes with.