In that case, what sort of limit is there on building it?
You mentioned "crate" once but no real explanation. If you only get one use people will start using it for TF2-like games anyway and lose all the ability/tactics in re-locating it or building offensively where a turret gun gets destroyed after one or two kills.
Possible idea: Multiple items for different uses in both prolonged attack/defend games and ones for your T/T deathmatches (single-use powerup style)
for example
BuildPDA
- You have 100 metal, regenerates 2/sec
- If you decide to make the other buildings too then it replaces the brick slots with options
- Otherwise just add it as a Wrench alt-fire taking out a toolbox for normal sentry
- Normal buildings start at 0HP
Wrench
- Used to repair buildings and speed up construction
- Otherwise like one of your normal melee weapons (maybe 3-hit kill instead of two)
Toolbox
- Pickup lets you deploy a Gunslinger-style Mini Sentry but only once
Normal Sentry
- Costs 60 metal (you can place another in 10 sec but only once, if it dies again you have to go defend yourself for 30)
- 6 shots/sec dealing 15 damage
- Wrench hit uses 20 metal to load a rocket (default one or less damage)
- No upgrades past that, Blocklanders don't team well so taking it out is hard enough
Mini-Sentry
- Ammo containers removed, alarm light on top, smaller
- Half health
- Builds faster and starts with full HP to use in battle
- 6 shots/sec dealing 10 damage
- No rockets