engine seized up?

here's an odd one- 1988 300sel, 98K miles- Mobil 1. I let it sit in my garage for about two weeks, then started it... heard what sounded like a fairly load tappet noise, so I figured one of the lifters may have bled down.

Drove it for about 25 miles, good oil pressure... as I'm pulling into my garage, it refused to idle, and I had to restart several times to park it. At this point, I suspect that the battery is shot, and maybe voltage is too low at idle. Next day, I buy a new battery, go to start car, and I hear the solenoid click, but the starter won't spin.

Removed the valve cover- nothing looks hung up. Removed plugs- nothing miscolored (I suspected sugar in the gas) Removed the drive belt (thinking that maybe one of the accessories had seized up)- nope, they all spin.

Anybody ever run into anything like this with the six? I change the Mobile 1 frequently, K&N air filter- car is near mint condition, and I just cannot understand how it could suddenly appear to seize up- any ideas?

Thanks- John Cazin