Thank you all for the advice. I forgot to mention that I'm in the UK and that I primarily use it for photo editing via lightroom and Photoshop and odd word processing and internet browsing, so a decent screen is a must, are the retina screens really good compared to my 2011 screen? I'll keep looking on the apple refurb website but is there anywhere else I should look too for refurbished models? I'll probably go for 2017 with ssd definitely, is it worth looking at slightly older model like 2015?
With regards to cloaning software, I use CCC as well as time machine, should I not be doing it? As a photographer I back up all my photos and system files once with CCC and other with time machine.
I can't imagine editing images on that old machine, and I had one. My father is still using it, and it's great, but it's now reached the end of the line. Every for his extremely modest uses. And I don't think it can even run Catalina. And perhaps you can get new camera raw updates, but not even sure about that.
In any case, a new machine will do wonders for your photography. Even without calibration the iMac screens are terrific, and quite accurate right out of the box. Loading images from SD cards or direct from the camera is speedier, and so is moving around images. Lr and Ps will work much better.
The visual difference is quite stunning. Go into an Apple store and take a look. Not only that, but you get much more of your image in view on the screen when editing, and with that resolution you can use smaller thumbnails and cull much faster.
Cloning software? it has it's uses. If you archive images before they're edited you can just use Finder copying; no need to clone those. But sometimes people conflate the real cloning process itself (which is different than just copying) with some of the features cloning programs like CCC or SD or ChronoSync offer, like scheduled copying, archiving, etc. I just let TM make copies of my image files; simple and free and robust. Different types of backups have different purposes; I myself only use clones for forensic purposes or with certain volumes where I want to preserve a certain state. Clones can be misused. Since they are an exact copy of say a volume, then they are an exact copy of the volume and its problems. So if you made one with the problem extent in it then now you don't have a backup, you have two copies of a wonky disk. But if you had a known good clone from say a week ago, before the problem arose, that would be fine, except for missing all the data since then. So like any backup the details matter.
But yeah, I'd go for a new iMac. Expercom.com used to have some decent refurbs, and MacSales some used machines, but I haven't checked in years.