Welcome to Benzworld. You are the proud owner of a W108 chassis. Mercedes labels its models by chassis. Unfortunately there is no W108 forum on this site as your car is too old to get its own place.
The W108 is, in the eyes of many, the most handsome post-war Mercedes sedan. While the 250S is the least powerful and least luxurious W108 model, it is still every bit a Mercedes. Your car is also the simplest mechanically, which makes repairs relatively simple.
To address your questions directly, your power steering pump is likely leaking. Could be a hose or the pump reservoir. Might be the belt. Seals and gaskets get old and brittle and start leaking after 43 years. Brake pedal going to the floor is most likely a master cylinder, but could be a leaking brake line. If the pedal comes back up, it's the master cylinder.
It appears from your questions that you are not particularly handy with automobile repair or function. I don't say this to insult you; the two problems you describe are very common and are perhaps the easiest to diagnose if you are familiar with cars. If that is the case, I encourage you to find an independent mechanic who works on older German cars regularly. The car you bought isn't particularly complicated, but it does need the TLC of someone familiar with the idiosyncrasies of a Mercedes.
$1000 is a pretty good price. You could part it out and get more than that for it.
If you plan to drive the car regularly (and with only 105k miles, it could be a daily driver for another 100k miles), I suggest you take it to the mechanic I mentioned above, and have ALL the fluids (brake, power steering, oil, transmission, rear end, coolant) flushed and changed. In that process have all the filters changed (air, oil, fuel). Please, please, please don't ever take your Mercedes to a bulk oil change place like Jiffy Lube and have anything done.
Have the tires checked carefully for dry rot, have the brake pads checked for wear, and have all the lines (fuel, brake, vacuum) checked for leaks. The shock absorbers and steering gear should all be checked as well. With the few miles (relatively) on this car, the parts should not be worn out, but may have age problems that argue for their replacement.
Parts are mostly available without extensive searching. Dealerships have most parts readily available although they may not be in stock. Dealers will charge more for parts and labor than after market suppliers like the fine vendors who sponsor this site. I buy all the parts for my R129 from after market vendors. Unfortunately my W220 is more complex and requires OEM parts except for expendibles like brake pads and filters.
Again, welcome to Benzworld and to the world of Mercedes ownership. You can find a lot of help in this forum, and if you google "W108" you'll find several owner's forums just for your car.
Oh, please fill out your profile so we'll know where you're from. Sometimes people will know a good mechanic they can recommend in your area.