The Craigslist bike is a fine bike. You don't need shocks though. they just squoosh up and down pointlessly and waste energy, unless you're on a rocky, rooty trail in the woods.
If you decide to buy a new Trek or equivalent, ask for a deal on helmet and racks. Then you can try on the helmet, which is key.
Buying from a real bike shop means you will get free tuneups for some period of time, and the bike will be professionally assembled.
Bikes from sports chains and department stores are assembled by . . . who knows? Sometimes competent people, sometimes not.
Aslo, the real bike shop will give you advice on fit and so forth.
Take your time if you decide to get a new bike. It will last for decades, as you say, will be fin, and will save you one point eight gazillion dollars.