I suspect you'll probably want to stretch a little above £100 and got for a GTX 1650 (~£130)
Anything new at ~£100 will not only fail to ignite the universe, it'll fail to play some 5-6 year old titles at 1080p low settings with acceptable framerates.
As for second hand, it depends of luck as to what's on offer. A 1050 ti for about £60 - if it stays at £60, which it probably won't - on ebay is an option. However, much more than that and you're sacrificing a fair amount of performance versus a 1650 for relatively limited savings, as well as buying something with no warranty. And private sellers on ebay don't always take the greatest of care shipping components...
The other issue is what's compatible with the PC.
The case looks like it takes standard length PCIe cards
. So I'll assume that's the case unless you say different
Does the PSU have a 6 or 8 (6+2) pin PCIe connector on it?
If it has an 8 pin connector that might open up the RX 570 as an option. if it has a 6 pin, maybe some other 2nd hand GPUs though tbh anything 2nd hand with only a 6 pin is likely to be so weak as to also not really be worth it. Maybe a GTX 960 if these go for ~£60 or less.
If it doesn't have a 6 or 8 pin PCie connector, that means you need the GPU to be powered by the PCIe slot, which means a 1650 or a 1050 ti. Also, these cards being lower power means less heat, which is probably just as well given the chassis.
Some old systems don't have a UEFI BIOS, while some modern video cards need there to be one. Sometimes a BIOS update will help, sometimes it won't / there'll be none available. Or sometimes you need only change a setting in the BIOS to allow for newer GPUs. To give you a flavour:
When I went to install it, I could not get a picture on my monitor. I was told I may need to update my BIOS. The computer is compatible with the graphics c…
Hi, I should update the bios of my desktop to mount a new video card, the Gigabyte GeForce GTX 1060. At the time the bios is the P01-A1 of 03/12/2012.