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TRD S/C if you can get one reasonably. Mine was $5200 installed in June 2015. Great gains in HP & Torque. It's durable/dependable which is why you buy Toyota. Good Luck!
If you can, definitely buy new. Due to the high resale value & low availability the used will cost just as much as new.
2wd vs 4wd: you'll have to decide your comfort level.
The Double Cab Short Bed (DCSB) will generally be avail in 3 configurations. SR5, TRD Sport & TRD Off-Road.
All 3 will...