Stepper motor for my DIY 3D printer
The size of the stepper is usually determined by the forces it must overcome on the machine (i.e. inertia). Obtaining a powerful stepper motor and then using expensive linear guide rails with high tolerances great axial rigidity is wasteful unless you are trying to achieve something specific (which it seems you are not).
Find the cheapest NEMA17 motor you can from a reputable site and start there.
Most of the work has been done for machines of that size, so you don't need to calculate what's required, you can just look it up online. A 250W @ 24V supply is commonly used on printers of that size. The majority of that power is going into the heated bed. If, however, your environment is so cold that the bed never gets to temp, no matter how long you wait, then it will be time to upgrade that bed to an AC mains heated one.