This depends on if the previous owner had an NFA tax Stamp for that particular weapon, which should you should have received along with the weapon as proof of prior registration as well as some documentation as to how he acquired it. You having any license does not make the Weapon Legal or illegal. Go back to the executors of the estate to see if such paperwork exists and at the same time see a competent NFA Dealer for assistance.

If it's a WWII gun it should have all of it's class 3 paperwork with it. Any class 3 SOT should be able to legally transfer it to you via form 4 + $200. If it doesn't have any paperwork I highly recommend deleting this post and consulting a lawyer.

Yes. in order to own a fully auto weapon you need a class 3 ffl license. In order to get that you must also check your state laws as some states wont allow class 3 license without being in business. You can also contact your local gun shops and they can help you sell the weapon legally. Hope this helps.