I've driven some 2015 Tahoes at work, and I've been around this forum for about five years and I've learned that you should be very wary of the 2015 models. Lost of first year glitches, from annoying little things to major mechanical issues.
That being said I personally own a 2016 Suburban and it has been pretty trouble and quirk free. I have zero complaints.
As far as "too much" mileage, well there really isn't any such thing as long as you understand that the price of the truck will be commensurate with miles. I wouldn't be scared to buy one at anything under 100,000 and I also wouldn't shy away from one with over 100,000 miles if the price was right.
Common mechanical issues usually consist of the condenser/trans cooler unit as noted, the transmission (the 8-speeds found in the higher trim level models are more trouble prone I have heard) which fails due to torque converters breaking down. There are complaints of electrical gremlins on the 2015's and complaints of roof panel wind buffeting issues that I can't remember were specific to the Suburban or happened on the Tahoes too. The AFM lifters are prone to fail which will be a costly repair. I have disabled my AFM system via a Superchips tuner so I don't have to worry about that again. You can buy a device called a "Range AFM disabler" that is affordable and plugs into your OBD-2 port that will disable the AFM system. Some of us here value absolute reliability over a few MPG's. Some don't and would prefer to gamble with their AFM still on. Educate yourself about the system and it's flaws and you decide. Fuel injectors go bad and cause misfire issues that can masquerade as AFM issues. My truck has had three (3) injectors replaced after they caused misfires.
My truck is a "lowly" LT model that I have spent some money on partially converting to a Z71. Tow hooks, stout aluminum skid plates, a mild suspension lift and more aggressive tires. If I had to do it over again I absolutely would have held out for a Z71, because there are functional, purposeful upgrades. Bling has never been my thing and I passed up on a few LTZ models when looking because they have un-needed tech that fails and costs money to repair. Some people "need" those creature comforts, though. That's a personal decision, and the LTZ packages are very whiz-bang neat-o, but I prefer function over form. The Z71 is the only upgraded model I'd ever pay for.
Good luck!
