To get the best price possible, having a tax return along is better than nothing, but that is about it. Although all the information on a financial statement is included on the tax return, it is not shown with the detail a buyer will want to see.
If you know in the coming years you plan on selling, get yourself set-up on QuickBooks now. Ask your accountant to help you set it up and review it quarterly, or at minimum at the end of each year. Being a being able to provide month by month financials, if the buyer wants to see your information in that format is a big deal. Having these reports available will add credibility and value to your business.
You may want to check with your accountant now and confirm he/she can provide monthly financial statements, when the transaction is in due diligence, it is too late. Do what you have to do, you will need to be able to provide month by month financials.
A P&L at years-end in an Excel worksheet is better than nothing, but not much more. You need financial statement prepared by an accountant or from of QuickBooks.