So what should I do whenever I purchase old lenses from 1980s to "Restore" (if that's the correct word) the whole item?
The easiest way is to know a good CLA/repair guy with reasonable rates.
Otherwise, you'll probably need a good service manual (or at least a youtube video), the right tools, and the right touch. Some lenses are easy to self-service, others are a bear, and you can easily damage/destroy a lens if you don't know what you're doing, or don't have the proper tools.
Hang out on vintage lens collecting boards, google a lot, and watch a ton of youtube videos, and get a sense of what's involved if you've never done it. This can be something similar to watch repair or restoring a vintage car, and you are talking about delicate glass and metal bits. How many small spanner wrenches do you own? Are you used to DIYing your own tools? Have you ever done any other type of vintage repair before?
If there's anything that I should checked before purchasing it, what are those?
Check whether the lens is compatible or adaptable to the camera you want to use it on. (See: Can I use lens brand X on interchangeable lens camera brand Y?).
Check the overall physical condition of the lens: how battered is it? Does it look to be in good condition? Is anything missing, loose, or damaged?
Are the rings smooth? Is the manual focus working? Is the aperture action clean and snappy? Does the aperture work correctly both in changing the aperture setting, and in open/closing for taking the image? Old lenses also mean old grease going gummy and sticky. This is the main reason older disused lenses may require disassembly and reassembly--to clean out old grease, particularly if it's causing the aperture leaves to stick.
Is there fungus in the lens? Fungus is typically seen as "totaling" a lens. Fungus waste product tends to be acidic, so it will etch its way through the coatings given enough time, so the element may need to be replaced, or re-polished and re-coated (likely to cost far more than the lens). Minor fungus may be cleanable and acceptable, but in general, fungus is one of those things you don't want to mess with.
Is there element separation? Elements are sometimes glued together, and that glue can yellow or weaken over time. Separated elements are going to be as bad as fungus when it comes to totaling a lens.
For more purchase advice, see also: