The ASUS Prime B350-PLUS AM4 motherboard is not compatible with an Pentium G4560. You need to get one with the LGA1151 socket.
You will also have to make sure your RAM is supported by the motherboard, since DDR3 and DDR4 are using different slots. But the G4560 does support DDR3 and DDR4.
But as @JDE already said you should check the local used markets. On ebay Kleinanzeigen you can find CPU/motherboard combos for a decent price.
For the PSU, a 400-500W unit should be totally fine. The graphics card draws at its peak around 120 Watts, the G4560 would draw 75 Watts maximum. So there is left enough room for all the other stuff. You can also find some decent priced PSUs at ebay Kleinanzeigen.
To use your notebook as an external monitor is not possible as far as i know. But since you have a Laptop; did you think about using that one with an external GPU. If you have an Thunberbolt 3 or TB3-compatible USB-C connection it may be useful to put more research into this topic.