Find colo with 24/7 access, high up time (good networking, multiple connections to the internet from independent providers and proper setup for working when there's no power) and "remote hands" in case you need to manually reset server.
Rest of the questions you have to answer yourself. Initial server cost will be way higher than VPS plus maintenance and paying for colocation is also not cheap. Servers will have to be upgraded every ~5 years, depending on the scale and you have to buy machine for the worst case scenario, there's no automatic scaling so if you sometimes need 32 cores and 1TB of RAM you have to buy that even if 99% of the time it will sit at idle.
I would rather find cheaper VPSes for non-critical systems or work on optimization of current solution.
Rest of the questions you have to answer yourself. Initial server cost will be way higher than VPS plus maintenance and paying for colocation is also not cheap. Servers will have to be upgraded every ~5 years, depending on the scale and you have to buy machine for the worst case scenario, there's no automatic scaling so if you sometimes need 32 cores and 1TB of RAM you have to buy that even if 99% of the time it will sit at idle.
I would rather find cheaper VPSes for non-critical systems or work on optimization of current solution.