I would suggest looking for a reasonable quality 500W psu, something with at least 80+ bronze certification. The Corsair CX supplies are a good option that don't cost a fortune so say a Corsair CX 500 or CX 600 unit would work well (these aren't the highest quality supply however I think it's a sensible option given the age of the machine).
Graphics card wise, the case looks to support full height cards and has at least two slots worth of space so you have plenty of options... I would suggest something like an AMD RX 570 might be a good choice as those can be had for a good price these days and would be good for 1080p, 60hz gaming (the newer RX 5500 series also worth a look, they perform similar and use less power but are often more expensive).
If you prefer nVidia, something like the GTX 1650 Super would be a good choice, or look out for a cheap deal on a last gen GTX 1060.
That would be great if his machine can take a standard ATX power supply.
That said, I do believe that the Corsair CX models that end in "00" are the older, green label models, which are to be avoided (not sure if any ending in "50" are also green label). Ideally, when getting a CX, do the lookup using PCPartPicker and make sure that they're designated as CX (2017). Note that CXM is different, and will probably be labeled 2015.
With a 2017 CX, let's say you even consider an RX 580 - a 185W card. I'd suggest at least a 550W PSU. If you were going with, say, a Corsair TX or RM, or with a Seasonic Focus or Prime, it's likely you could get away with even a 450W. Also, while the RX 580 is a great card for 1920x1080 @ 60Hz, the lower prices of the slightly lesser performing RX 570 usually make the RX 570 a much better card in terms of price-to-performance ratio.
Now for the potential problem - I know that some Dell models from the Sandy Bridge era simply WILL NOT WORK with AMD's RX cards (and quite a number of R9 cards), due to the use of legacy BIOS, and AMD's newer cards not supporting VESA Mode 103, which the old BIOSes with the old-DOS style look require. For example, I know my old Dell XPS 8300 (Sandy Bridge era Intel CPU) would not work with my R9 285, but my Dell XPS 8700 (Haswell era Intel CPU) does work with it. I have seen on the forums that the 10-series Nvidia cards will work with that old XPS 8300.
I do NOT KNOW for certain if OP's Optiplex (Sandy Bridge CPU) falls into this category or not. This will require some searching of Dell's forums to see if other people have had experience using one of the newer cards on the same Optiplex model.
If it
is a problem, though, then you might have to stick to, say, a GTX 1050Ti, or GTX 1060 (preferably 6GB, not the 3GB version).
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