You wouldn't necessarily go into their billing system right away. You need a review sheet of costs and profits. The person selling the hosting company to you will provide you with the breakdown on active clients, outstanding invoices, monthly recurring costs and income etc.

When buying another company there's no real way to get a feel for them without exposing the books 100% so you know exactly what you are buying. Failure to do this is shooting in the dark. In the interest of full disclosure, you have to have accurate books reviewed by accountant, business partners and a lawyer to be sure you're getting what you want.

Smaller companies, maybe not so much, but you still need to get in there and see exactly what is going on.