A M8 with cracked LCD is better than a M9 with cracked sensor filter glass ;-)
The price is the only measurement, to judge this.
Fully functional M8 are readily available at already quite low prices, new deals obviously can be had as well (see above).
The approach of getting the camera and having it upgraded with warranty makes sense (if the price is right and doesn't exceed total costs of a M8.2).
There has been a report from a buyer of a second M8 from Australia, if I remember correctly, who bought a camera with slight mark, to have it shipped to Leica, which have informed him, that the camera's housing was distorted, resulting in hefty total overhaul costs.
These kind of things are at the risk of the buyer obviously and should be reflected in appropriately low purchase cost or in a share of costs agreement with the seller, should further damage be found during a check.