I’ve not had to connect an external monitor to my 2015 iMac before, but I do know that the connection between my M1 MacBook Air and Dell Monitor can be a bit temperamental.
I can get a monitor with either HMDI or Display port (from my workplace) and could of course buy adapters but would rather keep costs down, but not at the expense of a solid connection.
Is there a preferred method of connection for these older iMacs?
Through Thunderbolt 2. Which means you’ll need to get at least a cable, which means that an adapter is indeed the easiest route. But it works great. I’ve been running a TB2 to 3 adapter on my older 2015 5k iMac and ran another 5k screen with absolutely no issues for years.
You can get TB2 to HDMI or DisplayPort adapters for very reasonable prices, especially if you’re not paying for the screen.
I’ve connected my two Dell 4K monitors to my late Retina 2015 iMac (27" in my case) via Thunderbolt 2 using both DisplayPort and Mini DisplayPort adapters. Both worked well and I expect this would be the simplest and cheapest route.
If you opt for HDMI, make sure that you’re using an adapter and monitor that supports 4K at 60Hz. I don’t recommend 4K at 30Hz; among other things, scrolling is not very smooth.
Oddly enough I’ve never used Thunderbolt 2 before. Is there anything to watch out for when buying a T2 to Display Port cable? This one describes itself as 'mini display port to display port (Thunderbolt 2 compatible)?Is that suitable?
Yes. The Thunderbolt 2 port on your iMac also functions as a Mini Display Port.
I’m not familiar with that brand, but the reviews look good and the price seems reasonable.
On a side note, both of my Dell monitors came with cables. Assuming the cables haven’t gone missing, you may find that you already have the cable that you need.
So does that mean that I only need a mini display port → DIsplay port cable, or does it have to say Thunderbolt 2? (sorry, a bit confused by the jargon)
Just a mini Display Port to Display Port cable.