Pretty much any used display will have an AutoTrac activation that's lifetime. 2600 or 2630 you had to buy it originally but once bought it was on there. 4640 it's in base so every one has it out of the box. Nearly every one you see will have AT on it since that was the most common thing used.

People get lazy and list a display with "SF1" or "SF2" which actually should read AutoTrac SF1 or AutoTrac SF2. Just the permanent autosteer that's on it. 


The receivers like SF3000 or SF6000 all get SF1 for free out of the box any time all the time. This is fine for tillage. They can have extra activations for SF2 (on 3000) or SF3 (on 6000) but you still have to pay for 3 months, 6 months, etc to actually use that higher level signal. For tillage, not worth it. 

So find whatever 3000 or 6000 you want and don't worry about getting one with an activation. Screen just make sure it has AutoTrac, any level.