I just wanted to get your advice for using/purchasing a grammar checker software.
* Not a teacher
If you don't have a decent knowledge of English, no grammar checker software is good. Why? Because it is a piece of software and it correct your texts following some patterns which don't and can't work smoothly on whatever text you input. No software is perfect. And, maybe not always, you'll get a messed up text. But you can still use a grammar checker to correct your mistakes if you wrote something in a hurry: spelling mistakes, subject- verb agreement, misused or unused commas etc.
As a piece of software, the only one I tested was WhiteSmoke. Look it up, test it and tell me what you think.