Congrats on the purchase. If you like to DIY, this car is a great project.
For the slow start, check the boost line to the alda. It's a clear line that goes from a banjo-bolt on the back of the intake manifold to a switch on the firewall and then to the alda, mounted on the injection pump. Many times, these become clogged and stops the boost enrichment.
Climate control may be as simple as a fuse. But it is likely to be more. Here is a link to help sort some of the problems out:
Mercedes-Benz 126 Service Manual Library
Brakes sound like a possible vacuum pump valve. There is a rebuild kit available that contains new valves.