We migrated from Windows to Mac at work a year ago and made the move to iWorks. Pages and Keynote are probably both better than Word and Powerpoint, but Numbers isn't a patch on Excel IMO. Lots of strange niggles that suggest the package just isn't as mature and well rounded as Excel. It's there because an office suite has to have some sort of spreadsheet functionality, but really Numbers doesn't fit with Apple's airy fairy creative emphasis and it's very much the poor cousin of iWorks.
iWorks will save stuff to .doc/.xls/.ppt and will play nicely with MS formats.
Depending on the nature of your work though, you may be better off with Office for Mac, or just look at LibreOffice which is free. You need to factor in the cost of new software into the buying cost of a Mac Mini, compared to just using your existing copy of MS Office on a new Windows box.
As with all Apple products, you're paying a premium for a Mac Mini, and it's a bit underpowered if you're comparing like for like. You also lose the pretty aesthetic value of the Mac if you're pairing it with dodgy old Dell peripherals and a screen that doesn't do it justice!
The thing is, if you spring £520 on a Mac Mini, or £609 to up the RAM to 4GB, and then consider the extra cost of a new monitor to do it justice, you're already spending the best part of £700-800.
In which case, you're almost as well looking at a 21.5" iMac for £999, which has an awesome screen, includes an optical drive, comes with the 4GB RAM as standard, as well as a beefier CPU, beefier graphics and includes peripherals like the gorgeous brushed-metal keyboard. The Mac Minis make very good home servers for media work and playing stuff to TVs, etc, but considering the absence of any monitor of peripherals, they're very expensive for what you get. Yes, it's more than you need because it's a better machine, and obviously a grand is a lot to spring for an iMac, but you very definitely get your extra £400 worth compared to a Mac Mini. The whole package is much better value compared to a Mini which isn't very good value at all really.
I would say ignore the Mac Mini. Go the whole hog for an iMac, or replace the Windows box with a new one (£350 max) and keep your old peripherals and software.
- They're £927 on Amazon at the moment which is odd as the price you pay is normally set by Apple. Unusual to see new Macs discounted like that.