I agree 100%, to make things more challenging and squirrel hunting more fun for the past few years when I do go squirrel hunting I only shoot if I get head shots. With a good scope it is very do able and I don't ever have to worry about gutshots.For squirrel hunting, when you may only get a head shot at a small critter 25 yards away up a tree, I vote for a rifle.
But even though a bolt action is generally more accurate and quieter, you said you want it to be a fun gun for plinking, too.
So make it a semi-auto .22 rifle or carbine.
I have no experience with the S&W MP-15, but it sounds like a good option.
Any gun with a factory-threaded barrel will save you money.
My host rifles for my AAC "Pilot" suppressor are both Ruger 10/22s. And both of them had to have their barrels done by a gunsmith.
I really wish Ruger offered a 10/22 with a factory threaded barrel, especially if it were a 16" barrel. They'd sell like hotcakes.
Sure, you can buy an aftermarket 10/22 threaded barrel. For $200 or $300. That's a budget-breaker.
This is me "hunting" (staged photo for GON article) with my 10/22 with the 22" Green Mtn. threaded barrel and the factory standard wood stock.