Borderline (expensive) foundation problem

[Matthew Samson]

We plan to turn our current house hack into a long term rental. We just discovered a crack in the concrete slab foundation that is a borderline major problem:
- It is a vertical crack about 1/8 inch wide that is at least 9ft long, which is causing a 3/4 inch drop in the flooring and some evidence of old (luckily) limited moisture and termite intrusion. 
- BUT, it also appears old and probably a symptom of the original house settling. It has not caused any noticeable structural issues. We know that some settling is inevitable on all slabs. 

The options we have are: 
- Do nothing and keep monitoring the crack.
- Seal the crack only from above and hope it stays static.
- Stabilize the foundation with helical piers on the exterior. 

The helical piers system would cost $25k but is a permanent solution. Is it overkill for a potential non-problem? Would the fact that our house is in the ~ $850k valuation range influence your advice? 

Even if you don't feel you have enough info to give me directional advice, how would you go about deciding whether to address the problem?