Hello and thank you for your question.
There is no standard version or "most recently updated version" as you request. While there are some examples floating around the internet that I will show you below, you must use these at your own risk. Of course there are companies that offer sample engagement letters that are periodically updated, but these may not be reproduced or distributed (like me, you would have to pay for them, so I can't just give you mine). Further, the Code of Professional Conduct, SSARS, etc., do not lay out specific standards for payroll services. You'll note that the Code of Professional Conduct's general standards might still be applicable (so, for example, client confidentiality rules are still applicable among others), and you should be mindful of independence issues in particular which are probably the most important aspect of your engagement letter for purposes of adhering to professional standards (see Interpretation 101-3, which clarified former ET Section 101.05 [now 1.295.040 generally], as you should be intimately familiar with it if you are performing payroll services for attest clients).
The following links may be helpful in formulating your engagement letter:
Here is a sample letter that picks up on many of the non-professional standard related issues:
You'll find some sample language if you go through the following:
This checklist is great for non-attest services in general and picks up on some things like arbitration clauses:
I use PPC guides, which do a nice job of covering what should be used in engagement letters, but there are of course others. Ultimately, you may want to run it by your attorney though too, as there are some legal issues that can pop up, especially with payroll because there is a lot of room for error and liability. I hope my time spent on this is somewhat helpful to you, even though I realize you just wanted a sample letter you could simply use.
Best of luck.