First, do you want to buy a new iMac? If you do, the screen sizes are 21.5 inch and 27 inch. Both models have a Retina display. The 21.5 inch model comes with a slow, 1 TB 5400 rpm hard drive. The 27" model comes with either a faster 1 TB, 7200 rpm hard drive, or a 1 TB "Fusion Drive". This link has a good explanation of the features, differences, etc. for these various new iMacs:
http://www.everymac.com/systems/app...s-between-imac-aluminum-late-2015-models.html
What version of Office for the Mac do you want? Office 2011 works fine, but "eventually" Microsoft will no longer support it. Office 2016 is fine also. I have both, having recently "moved" from Office 2011 to Office 2016. I am not a "power" user, so I don't see many differences. I also use Outlook for my EMail needs, which is included in either Office suite. The Mac OS comes with Apple's EMail program Mail, and a number of folks on here use it. But myself, I prefer Outlook.
The other thing you need to invest in is an external drive, as backups are critical. As for backup software, the Apple OS comes with Time Machine, but Time Machine backups are not bootable. Myself I use SuperDuper!, which creates a bootable backup, and I find it much more convenient/easier to recover from a SuperDuper! backup. (Carbon Copy Cloner is a similar, excellent product).
Finally, it is prudent to perform disk cleanup/maintenance/repairs on a "periodic" basis. There is Disk Utility that Apple provides as part of the OS, and it does a "good" job. But, I prefer a third party program called TechTool Pro, which has a lot more features. I also have the "ultimate" disk cleanup/maintenance/repair program Disk Warrior. There are also a couple of excellent freeware programs that are useful too.
In any event, welcome to the Mac world, and enjoy! And, don't be shy about posting questions here. There are a number of folks here who are more than willing to help.