To do DisplayPort you need something that will support USB3.2 Gen1 or better because Alt Mode is contain in that specification. I had done research about two months back and most solutions that will KBM, several USB3 accessory ports and connect two computers (by USB Type-C connectivity) to a monitor was going for about $200. The bandwidth to support Alt Mode means you should find something that has more then KBM+Monitor ports.
But if budget is a strong concern then something
like this might be a solution. I just found that on a simple search. I am not recommending it as I've not had it to test. still looking myself.