The newest iMacs are very difficult to DIY mod; about all you can do is add RAM.
There are arguments about whether adding an SSD or new drive voids warranty, and in the past I would have argued that perhaps it didn't. But with the new iMacs you can introduce changes that affect fan use, heat, etc. that would put Apple on pretty firm ground in denying a warranty claim, if it came to that. And it's easier to break these.
But macsales, for example, will do an upgrade on your machine and warranty it. But even they only do SSDs and drives and an eSATA port, and you can do some of that by ordering your own from Apple.
Rob