You are right that there is not a lot of data out there and my understanding is that no one knows if the 2018 will be the 6th generation 4runner or not.
Purchased a new 4runner in Sept 2016 and have been reading threads on a 4runner forum since (www.toyota-4runner.org
There is a pretty good thread on that forum comparing the 5th generation 4runner to the 3rd generation tacoma - which has the engine hypothesized to go in the 6th gen 4runner that might be an interesting read.
I am in the NE and unless things have changed, the dealer had a single 4runner on the lot and they had to do a "dealer swap" to get the one I wanted and Toyota was not offering any financing deals or specials on it.
All that being said, I would buy now. If the 2018 is still a 5th generation, then there will really be no difference and you have the benefit of knowing all the "kinks" have been worked out. If the 2018 is the 6th Generation, then you might
have to deal with new generation gremlins.