Dell Optiplex 790 help with internals

Everyone, I haven't done much in the last 10 years with computers so I am quite rusty but I will try to be specific as possible. My nephews goal is to turn this into a gaming PC and the tricky part will be this. He has a tight budget, in fact it is $130 (to cover parts 2 - 4) and I really want to support him. I have been searching and I think I can make this work but if I could have someone just glance at what I am purchasing, that would be wonderful (thanks in advance). He will receive the following:

Dell OptiPlex 790 - MT - Core i5 2400 3.1 GHz - 4 GB - 128 GB SSD

part 1: The RAM - I think I bought this properly and it was Crucial 4GB PC3-12800 CT51264BD160B. There should be a total of 4 slots and this should be exactly that. However, I would love to learn if other RAM is compatible just to learn or research for the future.

part 2: Power Supply - I looked at different power supplies, I was getting a 650W power supply since that will power most graphics cards and we are open to most. The PSU I was going to get was this on ($26 on ebay) https://www.ebay.com/itm/Brand-NEW-Hercules-DUAL-FAN-600w-Max-ATX-Power-Supply-SATA-20-4-pin-6-Pin/323617420723?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649. Would something like this with the optiplex that have the proper connections? If not, could you explain why or reference something budget friendly?

part 3: Graphics Card - we are totally open on this. All my nephew wants to do is run CS:GO, minecraft and fortnite. I believe a comfortable area for all 3 would be around 60 FPS, we are really open and I have seen a few posts with GPUs. I have looked at what was referenced in another forum for using a GTX 1050/1060. I wanted to snag the RX 570 (although it may be overkill) for around $80 - $85 used. Would that be compatible?

part 4: Hard Drive - As most of the budget is currently allocated, I believe a HDD would suffice unless someone anyone can give any links for something cost effective. I was going to buy a HDD 1TB 3.5" Desktop Western Digital SATA for about $15 - $20 on ebay. As long as it is SATA, would that work?

For those that are reading this, I sincerely appreciate it. I would love advice and feedback