There are pros and cons to using the d20 system for Ghostbusting. One advantage is that there are lots of monsters you can use. With some small adaptation to levels, you can easily modify existing D&D monsters.
The problem with the d20 system (and most other systems actually) is that it's designed to be a combat system, with feats and skills designed for beating the hell out of a monster (or it beating the hell out of you). The things particular to Ghostbusting would have to be drawn up almost from scratch, like ghostbusting equipment for example. There are some feats you can use (certain physical ones, like dodge), but most are particular to fantasy combat. You'll get some more if you lift some from the d20 Modern system or Weird War II's system, but even then, you're prerequisite system is all broken up.
Any system you adapt, there's going to be a lot of work involved. A lot.