Q) What info would a buyer need in order to make an offer?
Unsolicited offers rarely start out as offers. They start as conversations. Cold inbounds from potential acquirers are usually done to establish some facts and also willingness to entertain an offer. They need to have enough information to form a rationale (i.e. explanation as to why it makes sense to the acquiring company) and enough confidence that there is interest by the seller.

Q) Would that info need to be verified or can an offer be made on the condition of verifying later (if the conversation gets serious)?
A) Offers are always conditional. There is a delicate balance between knowing how much to disclose and when to disclose information, versus trying to force more commitment on the part of the potential acquirer.

Q) What is a general rule of thumb (formula?) for how to valuate a SaaS business?
A) Depends on the size and business of the acquirer. The smaller the acquirer (especially where its valuation is $100m or less), the more it becomes a relative valuation argument (what do we have and what do you have), but the larger the acquirer it is typically a talent acquisition model where the business dynamics are less important to what the team has demonstrated it can do and the perceived value that that team can make internally. This is even more true when the target (you) is generating less than $5m ARR on a trailing basis.

Finally, you phrased this question as "how would one go about entertaining an offer to buy my company?" so let me speak to a few general rules.
1) If you're willing to sell, be polite, efficient and courteous to any potential interest.
2) Quickly qualify who you are dealing with and their ability to make a decision (are they junior and just doing research or are they VP Corp Dev?)
3) Quickly establish mutual interest in a desire to dive deep, and get them to explain their process including other decision-makers etc.
4) Ensure that you have competent legal counsel who has significant experience in M&A, ideally with the buyer you're talking to.
5) *GET A TERM SHEET*. You have nothing until you have a term sheet and even then, you don't have a deal.
6) As soon as you have a term sheet, begin aggressively marketing your Company to other potential acquirers.
7) Try and put the impact of the financial outcome aside for a moment (very hard to do) and begin evaluating your suitors based on who you really want to work for over the next 3+ years. Do your due diligence on this question as much as possible.
8) Don't take your eye off the business or celebrate the deal until it's done. I've seen too many friends celebrate prematurely only to see the deal die or radically change at the last minute.

Happy to talk through this in a call with you in more detail.

Tom's answer is excellent. I would add: They are approaching you, which means you have an advantage. You have every right to ask (at the appropriate points, of course) why they're interested, who they are, what deals they've done, how they're capitalized (because if they can't close, their words are useless), what their forward strategy is, and what their philosophy is on founders and employees. Now, obviously, if MSFT, IBM or any well-known company reaches out to you, some of these questions (e.g., capitalization) would be insulting, but you have to sort the wheat from the chaff. It's easy to hear from unknown parties (who might be well-capitalized, respected PE firms or fly-by-nights). When Facebook offered offered SnapChat $3B cash, that was pretty straightforward -- nevertheless, they rejected the offer. You, too, can say "Not today, but tell me how you work because it's possible in the next couple of years." Also keep in mind the nature of your business: standard CPG manufacturing companies are almost always valued in multiples of EBITDA (or similar measures), while tech/social media companies are valued in terms of multiples of revenues or free cash flow, market share, or sheer numbers of users. There's no formula for SaaS per se, but clearly revenue and user numbers are critical. Look at comparable acquisitions -- i.e., in the SaaS space. If you have a clear #1 brand, studies have shown you're going to be worth 10x the #3 brand (the #2 will be around 3x, give or take). Last, never concede so the other party "likes you more." I've seen more negotiations undermined by a major, unilateral, utterly unnecessary concession (due to neurosis, fear, perceived intimidation, etc.) than you could imagine. Good luck and should you wish to discuss, I'm happy to do so.

There are boutique investment banks focused on tech startups (at different levels of annual revenue) that specialize in working with founders to help them identify their strategic options. Here are two examples of such firms. Some of the case studies on their websites can help you understand the process better.
http://vistapointadvisors.com/
http://arboradvisors.com/