You can't likely know which IP is fresh until you test it with a certain volume and check for deliverability.
The costs for delivery service aren't really high, not sure what the point of sending it all by yourself.
You should at least use a long living domain to increase deliverability.
> anyone in the company has experience with setting up email systems
I could very easily see someone spending $500/mo of their time, or a couple of days at most, fiddling with the mail system every month or dealing with crises.
As long as it isn't spam and bounces are handled appropriately deliverability shouldn't be a problem.