Just avoid these cars with "worst of the worst" reliability:
.
List of Used Cars to Avoid Buying by Consumer reports. 

"These 2006-15 models have a record of below-average overall reliability. ... they’re models with much-worse-than-average overall reliability based on multiple years of data. We recommend skipping all of them"

The Worst of the Worst Used Cars

CHEVROLET
Avalanche, Camaro, Cobalt, Colorado, Corvette, Cruze, Equinox, HHR, Impala, Malibu, Silverado, Sonic, Suburban, Tahoe, TrailBlazer, Traverse, Uplander

BUICK
Enclave, LaCrosse, Lucerne, Rainer, Regal, Rendezvous, Terraza

GMC
Acadia, Canyon, Envoy, Sierra, Terrain	, Yukon, Yukon XL

CADILLAC
ATS, CTS, DTS, Escalade, SRX, XTS

PONTIAC
G5, Montana SV6
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http://www.consumerreports.org/used-car-buying/used-cars-to-avoid-buying/