What is your job? Do you have exposure to life cycled hardware?

Too long

You'll need inlet and outlet, not just outlet. Also, look at your server power usage too, drive don't generate newrly a much heat.

Doing it for the experience and lab of it? Sure.

Doing it to actually use, fuck no. Email is the most vulnerable part of any org. You need to have a proper spam filter, dmz, web domain, several DNS records, certificates etc in order for mail to even flow in and out. It'll just be a headache and there are so many free options.

If you want your own special email domain, get suite for 1 person at 12 bucks a month and have it all just work.

Why make it complicated? Throw an ssd and at least 8gb of ram, should be fine to run windows 10.

The first question to ask is why? Exchange on prem is nothing but a pending security breech. Especially at a small business without dedicated experts and resources. You're realistically not going to save money either with how cheap hosted email is in comparison to a virtualization stack underlying storage, and monthly management time. You'll also have less uptime than Microsoft.

Coming from someone who was managing exchange on prem 6 months ago until I left that train wreck of an environment.