This should answer a lot of your questions. A lot of the answers depend upon the specific Mac, The 4G can only go as high as OSX 10.5.8
You will need to purchase the Mac version of Microsoft Offiice 2008
You can check Epons's site to see if they support OSX 10.5.8 and 10.4.11
https://discussions.apple.com/docs/DOC-2295 will answer most questions about moving to a 2006 or later Macs.
All newer Macs have built-in 802.11n WiFi starting in 2007.
Office 2008 is not the latest, Office 2011 is newer. http://www.neooffice.org/ and http://www.openoffice.org are free alternatives if upgrading to Office 2011 is found to be needed for certain things and you can't afford the upgrade right away. Several mobile phone options are available for Macs depending on which operating system you get. Check with your carrier.
If you buy new, all new Macs carry 10.8.3 or 10.8.4 minimum, as 10.8.4 was just released, where 10.8.3 was released back in March. iPads are not Macs, and come with some version of iOS 6.
Compatible software is listed on Roaring Apps when it comes MacBook Air, and Mac Mini. iPads use a different operating system, and have a different App store. Mac Mini has no stand alone power. MacBook Air and iPad have built-in batteries that can operate stand alone or plugged into the wall. The batteries are not easy for end users to remove under warranty. MacBook Air can take any screen that can adapt to DVI, HDMI, or Displayport, and so can Mac Mini. VGA displays are too low in resolution. While there are adapters for iPads, they aren't able to support as high a resolution for external display.
iPads also are sold as WiFi only, or WiFi and 4G/3G combined as separate models. The mobile iPads are specific to the carrier that sells them, or the store selling them and the country that sells them. There are SIM cards for alternate mobile carriers, as well as MiFi adapters that let WiFi connect to alternate mobile carriers as well.