Lots of people are reluctant to share close business secrets--but keep probing--he knows that eventually you have to learn all these things. You will not buy at any decent price, unless you have the business data--all the data.
There is a slight advantage to buying now and making the transition in summer or early fall. Customers are seldom shopping for a new company in the fall-the transition is easy. You don't have the problem of huge low-ball marketing pushes and thousands of competitive brochures and flyers being passed out to your newly acquired customers
The main thing you need to know is his prices and square feet. Naturally you will want to measure a few yourself--or at least with aerial photos. And you want to call a few to check and see if they are really customers or just names pulled out of the phone book. After that you don't really care about his chemical and labor expenses, or his overhead. Your expenses are the only thing that matters. Naturally do not buy accounts that are too far away to service profitably, (except at a big discount).
Naturally you will want to talk with some of the local fert dealers and see if his business reputation is good. Dealers are reluctant to release this info...but if the guy owes them money--the dealer is angry, and will usually tell you. If his account is current--they are usually happy to tell you that. Be nice to your dealer--he may have information that you need.
Purchase of his equipment us usually a spearate deal.
You may want a non-compete clause.
You may want to pay him over a period of 6 months--in installments with interest. Many banks will not finance your outright purchase. They want to finance equipment--something they can repo if necessary.