You can if it makes you feel better. That's why it's best to work with someone that is trustworthy and has a good reputation. But to answer your question, if it makes you feel comfortable, then by all means, yes.
Not a necessity...if you want to have an attorney involved - you might request the closing documents several days prior to your signing and have the attorney review these as well as the purchase contract several days before your closing date
An attorney will charge hourly. I guess the minimum cost would be $500. You should have read all the things you signed - the Loan Estimate, Closing Disclosure and Contract. IF you didn't understand something = ask before closing. None of the lender's documents will be typed to your wording.