I've used Expedia to buy tickets a number of times, only once or twice with Priceline, never with Kayak. The big advantage is that you get to see often hundreds of competitive prices, not just the few the airline sites directly show you. And often they are cheaper than directly buying from the airline.
Whether or not you can pick your seat in advance is completely up to the airline, not to the online travel site (eg. Expedia). Some airlines (eg. United, Alaska) do let you pick seats when you make your purchase. Others do not, these usually being airlines that charge for seat selection anyway. If your parent is elderly and has mobility issues, then I'd definitely call the airline after booking, explain this to them and get a seat assigned, even if there is a small fee.