First thing is to decide if he's an all out Engi, or if he intends to really split duties with other classes (I find that focusing on the engineer is probably worth it since it's the most needed class).
Body type can go any way; I have a light, a medium, and a heavy all with solid skills in the engineer section and I intend to add another partial engineer as well. The heavy is my only outright engineer. I say pick based on how you play, how you want to use your engi, and what weapons you want to have with you. Lights are great for blocking light only paths, mediums are a good mix and best for quickly getting to objectives (since lights tend to get places faster than their friends and heavies never get there), and heavies are good for defense and taking the extra few hits while repairing/removing at objectives if they can get there.
I recommend, if you are taking it seriously, taking all the engineer abilities. None of them are a waste for an all out engi (I can give more detail if you want it, or if anyone disagrees

). I generally don't take any of the top row of universal abilities (sense of perception, combat intuition, downed fire) and can skip silent running, but you should be able to get as many as you want if you're just an engi.
Turrets and mines are good on offense and defense. On defense it should be obvious where the choke points and objectives you are defending are, and the key to good turret effect is to place them somewhere that the player has to turn the corner to see them, thus catching them by surprise. Mines can go in doorways, at the bottom or tops of stairs, or a ways infront of or directly behind your turret to make it more effective.
On offense use the mines and turrets to defend side entries to areas you need to move through, or to defend the objective areas (as if you are a defender).
My wife is now dragging me out of the house so I have to cut this short, I'll try to come back and get into more detail, and add some stuff on buffing and positioning.