On the keyboard thing:
depends how does your keyboard connect, does it come with it's own blue-tooth port you connect via usb? If so it might but if you don't already have a keyboard you may want to buy a mac keyboard to go with the imac, there are also pretty good 3rd party keyboards that go well with it and cost the same as a Microsoft one.
On your question with iMac:
I've worked with numerous Windows os and made the jump to mac I think almost 2 years ago and I really like imac. It works really well and you don't need any security software as it's pretty much built in. I can recommend you to get an imac, however, there are downsides.
If you want to play games, there aren't that many for the mac and pretty hard to come by in stores, at least around me. Also mac isn't super compatible like for instance windows, so you'll have to buy special mac compatible stuff which can in some cases cost more than the Windows equivalent.
Software you'll have to get accustomed with open source programs and learn where to find the mac ones.
Mac has a few of it's own problems like the white screen of "F**k ME", but that can be fixed easily and is supposedly no longer a problem witht he current generation. But don't expect the mac to be the "be all end all" of computing.
Basically if you get rid of the standard mouse and keyboard the machine is pretty awesome.
You might have a bit of a shock when first using it and it will definitely take some getting used to, just go to your nearest store that has macs and try playing around with one of the "try me" ones (can't be bothered looking up the real term..) and also ask the clerks for advise on what you'll need depending on the purpose of your mac purchase.
I hope this was of any help.
