You have a tight budget so you want to buy a cheap bike and then get lots of hugely expensive alterations done?
Just no.
Either make do with the bike as-is or look for a bike that already has the spec you want. It'll almost certainly be cheaper to buy a bike that's already built to spec than to buy a difference one and upgrade it.
Upgrading to hydraulics brakes is a no go, you'd need a new fork, frame, and wheels in addition to the braking system.
Repainting is time consuming, so expensive.
Changing to an internal hub means a new wheel/hub and hope it fits the frame.
Changing to a belt means completely new drivetrain.
So if you're talking about changing the colour, brakes, drive train, grips, wheels, frame, fork, and saddle, then pretty much all you're keeping is the seatpost and handlebars.