What’s best bike for serious but broke beginner

[SkipdiverJohn]

I'd say near enough any bike that fits you and that hasn't got suspension will do the job - doesn't really matter whether it's road, hybrid or MTB. A bike is a bike*, and it's good to get you started..

Absolutely this. Riding any type of bike regularly is going to improve your fitness and possibly help a bit with weight loss, and it doesn't matter a damn what genre of bike it is, how old or new it is, or what quality level it is - so long as it works reliably.
Most of my cycling mileage, and therefore the level of fitness and weight that I maintain, will be the result of me casually knocking around on cheapo beater bikes that have basically cost me nothing and have been built up and maintained using salvaged parts.
All the short 5-10 mile utility rides and casual weekend leisure jaunts on an old hack MTB or steel hybrid add up to a lot more miles over a year than the miles I cover on my best quality and most expensive machines. The cheapo ones are heavier too, so I get more excercise when riding them.