Calculate debt capacity

Hi guys,

Quick questions for the pros ;) are there any good resources to learn about the various debt instruments and how to model them in a M&A / LBO context? also: do you guys know how to calculate debt capacity for a company?

We are offered a step down unitranche over a period of 7 years in USD. It is basically a bullet loan with step down interest margin depending on our Ebitda leverage. I would very much like to check how much we can actually afford given our BP and model the whole things so any info would be really appreciated :)

Thanks!