The 260E is basically the same as a 300E except with less engine displacement. The engine is the same configuration in most every way except bore and stroke. The only downside to the 260E will be slightly less power. Smaller engines are supposed to offer better fuel economy, but I expect that this advantage will be marginal. If the car has enough power for you and is in good condition, I would not hesitate because it is a 260.
I would recommend having any used car checked out by a technician that you trust.
Have a great day,