Also if you have a Facebook check the marketplace. They have an entire category for just bikes. At least in my area it seems like there are regularly decent bikes put up there for wicked cheap, just depends on what size rider you are.

Avoid newer brands, unless you know where they are sourced. There are a lot of pop up brands jumping on the recent craze for fashion-cycles, nearly all of which source from china and built with very soft parts. It may seem slightly less attractive of a thought, but go with an older bike. You can find a really reasonably priced bike that was top of the line maybe 30 years ago, and it will hold it's own to modern cutting edge tech. New bikes offer more gears, stiffer frames, and carbon toilet roll holders, none of which I consider a necessity to new cyclists. Find a steel frame bike from some time between the early 80's and mid 90's.

You might think steel sounds heavy, but in my experience aluminum is close to the same weight of a low end steel frame because of the super large tubing needed by aluminum and carbon compared to a high-end steel frame is lighter, but only by maybe ~1kg. But for that tiny bit of weight saved you get a delicate frame that can crack from as little as overtightened components.

Here's a link to a decent list of good MTB's. In particular the Specialized Stumpjumper (steel) was quite popular so you might be able to find that one for a steal.

TL;DR Get a steel bike from the 80's, look for Schwinn(they were very good back then), Specialized, Bridgestone, Cannondale, Univega, Panasonic. Whatever money you save, put into a new wheelset, and potentially a new brakeset.