Welcome to the forum ... a few tips for you.
The first thing you need to do is to find out what needs to be inside your pc to run a game you intend getting.
Example .. you mention diablo so i asume you mean diablo 4.
You simply type what do i need to run diablo 4 ... see what i found on the link on the next line.
Diablo 4 system requirements 2024 - can your PC run Diablo 4? What is your PCGameBenchmark rating? #pcgaming
www.pcgamebenchmark.com
If you click on the link it will take you to a site to show what you need inside your pc to run it.
Sometimes it will just show a list of parts and when you have a pc you can run a small program that scans your pc to see if a game will run . On diablo 4 link page on the left you will see a blue box , thats a mini program that scans your pc hardware.
I dont know how other games clients work with regards to refunds but i would suggest you get the steam games client , if you get a game that you dont like or wont work on your pc you can get a refund. The only refund conditions are you must have used it less than 2 hours or owned it for less than 2 weeks and yes they do have a way to check that.
Looking to the future ... if your pc breaks or you just want to replace it all you do is re-download the steam client , make sure you use the same log in details and password and when the client runs you will see all the games you previously bought but the titles will be greyed out , all you need to do is download them again.