Yes, authorized dealers still have some of the old R12 freon on hand, albiet it is expensive $30-50 a pound and the system takes 47 oz according to the Bentley manual.
If your AC hoses are still in good shape and your compressor is in good shape it is economical to continue to recharge with the R12 for as long as you can. But remember that R12 depletes the ozone layer so there is an environmental factor to consider.
Given the age of your car '79 the hoses and compressor may be suspect. To change to the R134 newer type coolant you have to change out your hoses and your compressor and reciever dryer - (not 100% sure if you need to change your reciever dryer) - expensive to do the entire system. The R134 is a smaller molecule and will seep through the hose of your original R12 lines. Note the R12 lines do have holes, so eventually you will need to recharge every system.
With regards to the horsepower question you won't get any more horsepower from the engine by removing the AC any different from when you do not have the AC running... ie when you turn on the AC you will notice a slight load on the engine, then when you turn it off the load is released. The AC compressor is belt driven from the engine. However there will be a weight reduction in the total weight of the car, hence most track race cars remove the AC. Another consideration for removing the AC and why I didn't remove it when mine needed recharging is the defrosting and defogging capability in wet weather.
There are many posts so do a search here.
Senior members I'm sure will chime in where I have left gaps as they really have some great knowledge in their posts.
Also there is a website:
http://www.griffiths.com/achelp/achelp1.html
Wayne's 101 projects for your porsche gives an overview of the AC system and the Bentley manual helps with troubleshooting and diagnostics.
Cheers
CrS