Time to update the buying guide for 2019. And even if Apple does release the rumored 16-inch MacBook Pro in October, all of the following will still apply for now.
I’m fine with someone buying a used computer as long as the service they go through has buyer protection (e.g. eBay). But refurbished machines from Apple or OWC, especially ones covered by warranty, are a superb option
Base specs for any Mac
Getting less than this is wasting money and, worse, productivity:
- At least 16 GB RAM (memory)
- Absolutely get Flash Storage (a.k.a. solid-state drive or SSD), and not a SATA drive or “Fusion” Drive
- At least 512 GB Flash Storage, though I prefer 1 TB for most folks
- Make sure it is covered by the AppleCare warranty if it is younger than three years old
Laptop models
MacBook Pros with Retina screens from 2014 or 2015 are great. And the MacBook Air from 2018 is also great. Do not buy a MacBook Air older than 2018 unless you get some kind of great deal on it ($400 or less) and it has at least 8 GB of memory. 
Do not buy any laptops made in 2016 or 2017, and I generally recommend against 2018 MacBook Pros, because the keyboard is controversial-bordering-on-tragic, and the Touch Bar is kind of gimmicky, and they are generally overpriced. But if you need one and you have the jack, a 2018 MBP is basically fine.
Do not buy any year of the 12-inch MacBook, unless you have very specific needs for lightness and don’t require a great deal of power.
Desktop models
Any iMac with a Retina screen is fantastic. If you’re a professional who works with images, get an iMac made since 2016 because the color gamut of the monitor is wider.
The 2018 Mac mini is one of the coolest Mac models ever.
If you’re a professional who needs an iMac Pro or the soon to be released Mac Pro, you know who you are and you probably aren’t reading this section.
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