my father in law needs a new setup, he wants to minimize the cost so I need to think creatively.
I'm going to build an ivy bridge setup and I'm thinking of reusing an old psu that I have, the issue is the connector, the old psu has 20 pin connection while the mb has 24, is it a good idea to reuse the psu or I should get an new one?
If you're just looking for something cheap but decent, the antec VP-450 is a great model (better than Corsair's CX series in some ways, but not quite as good as XFX) that can be found for about $30. However, I wouldn't solidify my recommendation without seeing the system's hardware specs first.
if you really want you can get an adapter but its not worth it and probably doesnt have enough power for everything anyways
Why would you get an adapter? A20 pin connector fits right into a 24 pin slot, just with four pins of the slot unused (the same is true for a 24 pin connector in a 20 pin slot, but with four pins on the connector hanging over the slot). An adapter would be worse because it's more cable to go through (slightly increasing energy consumption and decreasing energy efficiency and increasing heat generation) and more points that can fail.