Buying a 27" iMac

Hi,
I'm thinking of getting rid of my Mac OS X 10.7.5 and getting the iMac 27 inch with retina 5k display, The old one is getting rather slow.

I can see that I will be gettig a far better screen - what else will I be getting that is better?

Are there any issues to worry about?

I expect the screen will use more power when I'm using it - will it use more in sleep mode?

The shop won't part-exchange it. What are good ways of getting rid of the old one?
Is the extended warranty worth having?

You didn't say SPECIFICALLY what your older Mac is. "Mac OS X 10.7.5" is just the version of the OS, not a computer..

Either way, the new iMacs have been out for a while and any issues have likely been ironed out..

Depending on how old your old Mac is, you are getting a new computer and everything that comes along with it..

When the computer is asleep, it is using very little power..

You get 1 year of AppleCare for free with your purchase, adding on the 2 extra years is usually a wise thing before the first year expires. Compare spending approx. $170 on the 3 years of AppleCare to having to replace the Logicboard in year 2 for $900-$1100 or some other expensive component..