I took a look at some of the full motion, DIY home built as you've suggested. I'd personally stay away. Too many are made without proper structural support and either 'bounce' too much, or could fail resulting in damage to the occupant or equipment.
I'm in the simulation business and long ago the industry moved away from motion, not because it's hard or expensive (both of which are true) but because many studies have concluded the minimal gain you get in simulation from motion vs non-motion is less than improving the realism of the user interface, in this instance the cockpit of the car. Some will argue that commercial aircraft trainers run motion, and that's because of the relatively low g maneuvers they make. In a race car, as in a fighter jet the lateral and forward accel/decel can be much greater than the sense the addition of motion can give you. Personally, I would invest in creating a simulator out of the cockpit of a car, fully enclosing the occupant , and removing all visual queues they are in a room, i.e. enclose the occupants full field of view within the sim by placing monitors around their field of view. You'd be surprised how easy it is to trick your mind into 'feeling' motion when there is none when you remove all visual queues to the contrary...