If you're asking whether you need a lawyer to draft a contract, legally, the answer is no. Anyone can draft a contract on their own and as long as the elements above are included and both parties are legally competent and consent to the agreement, it is generally lawful.
What should be included in a buy-sell agreement? Any stakeholders, including partners or owners, and their current stake in the business' equity. Events that would trigger a buyout, such as death, disability, divorce, retirement, or bankruptcy. A recent business valuation.
The biggest difference is that an SPA is the sale of all shares, and an APA is the sale of selected assets. Therefore, they are both different transactions and have different procedures.
The biggest difference is that an SPA is the sale of all shares, and an APA is the sale of selected assets. Therefore, they are both different transactions and have different procedures.