Thanks for your reply  

The challenges I'm currently facing is that AutoCAD is slow and laggy, and it often crashes completely. I've linked up a second screen to my laptop, and sometimes I can't use it because of the lag. I often have to shut down everything else I'm working on. Ie I work in Photoshop simultaneously. 

"Are you intending to run the MAC version of AutoCAD on that imac27?
Or are you wishing to run WindowsOS on it then AutoCAD for Windows?"

This is the sort of thing I would need advise on  

I think I would prefer an iMac instead of WindowsOS Desktop as I've heard that they're better for designers. As mentioned, I use Photoshop a lot. Seeing that I've always been a Windows user, I'm just very uncertain how much different a Mac would be.