Buy if you need one now..... Wait if you don't
I bought my first computer when I came to the point that I needed one as I wanted to do most of my work at home. That was a 2005 Mac Mini. It had its shortcomings, but it did the job.
I bought my next, also a Mac Mini, four years later because I needed to replace the original.
The hard drive had failed. The power supply was dicky (a legacy of widely fluctuating mains voltage and no earth, and a fairly basic UPS that did not have AVR; both since rectified), and I would have had to buy a new copy of Office as what I had been using had been "cracked" by the shop when I bought it, and not paid for. All up repair would have cost about half the price of a new one.
When I did buy the new one, it was just the computer I replaced; my monitor etc were fine, and still are. That is one of the joys of the Mac Mini.
Three and a bit years on, the 2009 Mini was getting a tad slow. ! GB of RAM was not enough for my current needs.
I popped the Mini in to the shop for a bit of a mid-life tune-up. They slipped another 4 GB of RAM in alongside the 1 GB originally installed, so now I have 5 GB. They checked the HDD and reckoned it is fine. I got them to load Mountain Lion too. I don't have a credit card, so don't have an App Shop account.
For about an eighth of what it would cost to buy a new one loaded with equivalent software I have what seems like a new computer. It should be good for several years ahead.