You can guesstimate the oil level, but make sure to leave a lot of room to contain the oil. Or you could put the turkey in the empty pot and fill with water to just cover the bird....that' s how much oil you' ll need. Of course, you' d have to this before the injecting & drying process.
Boil over (or oil over in this case) can happen fast and be dangerous. A good way to get burned and start a fire.
As for the marinade, any of the off-the shelf ones will do a good job.