Welcome to the family of SL owners! I myself have a '75 450SL that I am enjoying very much.

You would be best replacing the turn signal lever assembly. Sounds like yours is worn.

If "best price" is your motto, go to Pick 'n Pull with your tools, pay $2 admission, and go find a W116 to raid. Most likely they won't have 107 SL's, but maybe you'll find a SLC or two. These cars usually go to the specialized recyclers. W116's are pretty plentiful and come in a variety of tastes. Here in town there's a 300SD, and that works just fine for several parts. www.picknpull.com

Steering: Replace the steering box. My SL only has 103K miles, so the steering feels pretty good. My former '74 SLC with 160K had a bit more play, so it all depends on the mileage on your car.

Tires: I don't know about using 60's but 55's would be nice. See below link (not my car!):

http://www.care2.com/c2c/photos/view.html/view/234/120428746/426829300/

Have you tried talking with your co-worker about the car history, or asking about the headlight condition? Perhaps you could find out if he had tried replacing the lamp at one point with the same result?

Enjoy your new ride!