wood
preferably hardwood
preferably do not put the letterbox in the front door, put it beside the door (hinge side) far from the lock, as it is possible to manipulate a key or handle through the letterbox with a simple home-made tool.
For the front door, fit a BS mortice deadlock plus a BS (preferably, otherwise double-locking) nightlatch, plus a chain
For the back door, a BS mortice sashlock plus mortice bolts top and bottom
If you buy your deadlock and sashlock from a proper locksmith or lock centre, you can have them both suited to the same key for a modest exra charge, this is very convenient as it saves having to carry, copy or mislay so many keys. I have my back doors and garage personal door on the same key as well.