arimidex is not much use to prevent gyno from deca as it is caused by progesterone, not estrogen. To prevent deca gyno you need to stack it with winny.
There is little you can do to protect your prostate. Unless you are over 40 with a prostate risk I wouldn't be too worried.
btw, $280 for 28 tabs is the most expensive arimidex I have ever heard of in my life. Why don't you contact the man in China instead?