Buying Used Cadillac Engines—Northstar Reliability & V6 Options

[yardsandparts]

Hello all, seeking some advice on purchasing a Used Cadillac Engine. I am struggling to decide between a reliable, later-model Northstar V8 (post-2003 fixes) or a modern V6 from a CTS. Given that I'll be buying a used Cadillac engine, what would be some critical red flags to look out for before purchasing, especially on the Northstar? I need a good, trustworthy supplier who actually offers some warranty on the core block and internal parts. I'd rather not have to buy an engine only to have it fail in six months. Any recommendations for the most dependable year/model motor for longevity?