I don't want to buy a whole set if I'm going to use only one out of the whole set and plus I only have to use it this one time.
Actually you mentioned that you need two sizes out of a set.
Assuming that you will use a general-purpose tool such as a screwdriver only one time in your life is ridiculously short-sighted. If you're game enough to take apart a computer today, you actually think you will never use it again because you have a short life expectancy?
Avoid cheap tools (such as "jeweler's screwdrivers"). Quality tools can last almost a lifetime, whereas cheap, low-quality tools can damage your work or wear out. There seems to be different types of screwdrivers. Home Depot, Lowe's and hardware stores seem to sell screwdrivers aimed at the carpentry/woodworking and automotive trades. Electronic supply stores seem to carry a different type of screwdriver. For example I could only find #1 and #2 Phillips screwdrivers with extra long 10" shaft at an electronic supply store.
Look for "hardened" tips that are a dull and a darker color (grey or black) than the rest of the (chromed) shaft. Tips that are chromed like the entire shaft will flake and round over under heavy use.
My fav brands are Wiha and Xcelite (Cooper Tools).
Don't use the wrong size (esp undersized) screwdriver, or you risk damaging the screw head.
You probably do not need to wear an anti-static wrist-strap, but at the very least you should always discharge yourself of static electricity by touching something metal (that is grounded).
A pair of tweezers or needle-nose pliers or a hemostat can be very useful.
Regarding magnetic tip screwdrivers: with high quality screwdrivers, you really don't need a magnetic tip.
The Phillips head screw will fit so snugly on the tip that the screw will stay in place by friction alone. You probably have to actually use high quality screwdrivers to appreciate them over cheap tools.