4pksr is faster and lighter than the 4pks. Worth the additional money? If I was buying new I would get the 4pksr over the older 4pks.

The 4pksr is much better than the 4pl. If you are serious about racing then the 4pksr is a good investment. It will last you many years as long as they are taken care of. If you aren't that serious about racing and are more into bashing you'd be just fine with the 4pls. Its a good mid level radio.

I'd only buy a used radio if I knew the person well and knew how they took care of their equipment. It's a lot of money to spend and it's not like you can just replace every part on it easily like you can a used RC car.