POSReady 2009 is just XP with the multimedia stuff stripped out of it. Reinstalling XP and applying the famous POSReady 2009 registry hack means it will receive the latest security updates except for anything not included in POSReady 2009 like Windows Media Player. Even MSE will have definition updates until April 9, 2019!
The main problem with XP is the last up-to-date web browser for it will be released in September 2017 as Firefox ESR 52.4. Slimbrowser using iE6's Trident Engine was last updated June 6, 2016, Chrome ended support in April 2016 after v49, and Midori using Chrome's former Webkit engine (until v27) was last updated August 30, 2015.
This is important because after Windows 9x had Firefox end support with 2.0.0.20 on December 18, 2008 it pretty much became unusable online.
Windows Thin PC sounds like the Windows 7 version of Windows Fundamentals for Legacy PCs which was pretty limited (couldn't install MSE or even Acrobat on it!).
If it doesn't work out for you, I'd suggest turning it into a Chromebook-like browser-only machine using Webconverger, or using a very lightweight Linux distro such as Puppy Linux due to the low RAM. Neither requires installation as they boot from the optical drive.