Fungus will typically affect contrast more than resolution. If it's severe or if the lighting is just "right" you'll see a drop in contrast. Fungus will also tend to increase with time rather than decrease if you don't do something about it. Whether it's "worth it" to buy a lens with fungus is an individual decision. Certainly if the price is low enough it could be a worthwhile experiment.

The only real way to deal with it is to take apart the lens and try to clean it off the glass. If it hasn't been there too long it may be possible to remove it without damage by simply cleaning the elements. The longer it's been there the higher the likely hood that it has etched into the coating or even etched into the glass.

Some people suggest that you can "kill" the fungus by exposure to UV light (even sunlight), but even if it does (which isn't certain), you'll still have it present (dead?) inside the lens and it will still lower contrast in some situations unless you clean off the residue.