Before you throw any parts at it, check the starter cable connection at the body side. The lug where the cable connects can become corroded. I've seen it happen many times.

If you're not sure what I mean I'll try to explain. The main battery cable (+) is connected to the front pre-fuse box, located beneath the front passenger's footrest panel. The other side of the pre-fuse box (where it passes thru the body) is where you want to check. You'll need to get underneath the car to find it. Just follow the big cable from the starter.