Whatever you do, get one with an SSD, or at least a fusion drive if at all possible. The standard HDD ones will make you bang your head to the desk waiting on it to catch up.
The screen will be a massive improvement over your 2011 iMac, but the i5 isn't going to be a huge leap over the Sandy Bridge chip in yours, but still a good bit faster and a lot more efficient, plus the Intel cards are going to be more reliable and even better than the AMD unit in yours.
I'd say go for it if you want one, just grab one with the SSD
5 years is a solid upgrade cycle too IMO. I went from my late 2008 MacBook straight to a late 2013 rMBP summer of 2014 and it was a substantial upgrade and well worth it in my book.