Answer: This question, as with so many others, really cannot be answered without first knowing the rarity of the car, the condition of the car, and its state of originality.
If the car is rare and is in excellent original condition, or has been restored to excellent original condition, any modifications that alter the way the car was originally delivered from the factory will have a negative effect on its value.
In reality most cars do not meet all of these criteria, and the installation of factory installed options will not decrease its value. In fact, if done properly, some options may actually increase its value, including those that you inquire about.
How you go about installing these options may have more of an effect on the vehicles value rather than whether you go about installing them. There are two ways that you can approach this undertaking. The first is with original factory parts, and the second is with aftermarket parts. By the time that you are done with parts and labor, the costs will probably be similar either way.
Using original factory parts will be significantly more difficult and time consuming because all of the parts needed to install these options will have to be identified, sourced, restored/rebuilt, and installed. This includes every nut, bolt, hose, bracket, belt, clamp etc. However, this approach will probably retain or enhance your cars value more than using aftermarket parts.
Using aftermarket parts is as simple as ordering a kit based on the year, make and model of your car. Complete kits are available for power brake/disc brake conversions, air-conditioning, and seat belts. The quality of the components is superior to that which the factory originally installed, but these same components do not exactly resemble the original components, and that is why it is possible that the installation would not retain or enhance the value of the vehicle as much as the use of original parts.
Installation of power brakes would cost about $350.00 for parts and approximately the same amount for labor. An advantage of this upgrade is that you would get a “dual reservoir” master cylinder which means that if one brake were to suffer a loss of hydraulic fluid pressure you would not lose all of your brakes. Most people upgrade to front disc brakes at the same time which adds about $1500.00 to the cost.