1990 300SEL

Well, I'm almost done renovating this car, but I can't figure out the hard starting problem. New plugs, air filter, all the obvious things. Here's what happens:
The engine needs to crank between 12 and 15 times before it will start. Otherwise, I can turn the key on and let it crank 2 or 3 times, turn it off, and then turn it on and it will start after about 4 cranks.

1. Which is harder on the starter - one long attempt or two shorter ones?
2. Whichever way I do it, after the engine starts, and with the heater/AC OFF, what I'm guessing is a heater flap will slam by the front passenger foot area. Often I will hear it slam again when I turn the engine off. Doesn't matter whether or not the heater or air conditioner is on.

Any ideas greatly appreciated.
Thank you.