Reinforcing the door and door-frame is a good start but if there are groundfloor windows without bars then the door will not be the point of entry anyway. A good solid door with a police-bar lock will hold against a very determined attack. Master lock sells one that props between the door handle and the floor - these are week and useless against a smooth floor. A good locksmith can sell you one which bolts to the floor and the center of the door and gets easily out of the way when opened. If you can install one of these you will be safe at that entry point. Make sure that the bar is a single piece of steel not some collapsable telescoping alluminium thing. Also make sure that the fixture bolts onto the center of the door and the floor no friction fits. this arrangement is very secure for the door. Again if you have only glass protecting your windows on the ground floor then you are wasting your money with anything stronger than a deadbolt on the door.