I dont think junk bikes really exist anymore if you buy from a proper bike shop and not a department store. You should spend what you can comfortably afford and leave enough in the kitty to change the seat if you dont like the one it comes with and maybe down the line if you ride a lot get some clip in pedals and shoes.

Look for features you like such as how the shift levers work - whether you prefer to use your thumb of you prefer to do it all with the brake lever for example. Buy a bike you think looks good, sounds silly but you want something you can be proud of

Then ride the wheels off it

having said that, if you can spend 1000+ it would probably be worth it