After checking temperatures, hooking up a PSU is the next easiest thing you can do to isolate the issue. No need to fully install it - just remove the panel, disconnect your current PSU’s connectors, and then hook up the replacement with the PSU sitting out front.

Not sure how similar it might be in Poland, but in the US some corps have very flexible return policies so I would ‘borrow’ whatever’s convenient and then return it after ordering what I actually want.

Alternatively, your local repair shop could probably do this all within 15 minutes.