Engineer's job is just to sit at one spot with his turrets and do AoE healing to your teammates. Mobility Items for him are just to give him some speed in the preparing phase since he does not have any mobility abilities.
This is the main item for the build. Since all Engineer's items are stacked onto his turrets, this allow the turrets to have a healing radius under them too. You can stack as many as you want to increase the radius. The radius of the healing circle is unlimited, but the healing is capped at 44 Bustling Fungus. This allows Engineer to heal both himself and his teammates in any fight. Since this item is not that useful to any other characters, it is good to build up.
This item reduces Engineer's cooldowns. He can place his shield faster and get his turrets up faster.
Also reduces his cooldowns.
This item increases his overall damage. Every attack is explosive, and it is applied to his turrets too.
This is one of the main item for his damage. Since his turrets are always going to be healing under attack, they will always be firing skulls and doing damage to enemies.
Proc damage done by both Engineer himself and his turrets. It triples his damage.
This increases his healing by 100%, just makes him do even more healing.
Gives him a extra life, but also gives his turrets extra lives.