Local Server Requirements

If you are using XAMPP, then only a 32 bit version is available so more than 3.2 GB RAM is meaningless.
You can however, use VMWare Player to run 64 bit Debian/Ubuntu/CentOS, etc., inside Windows, whilst all your end users can be on Windows browsers.

100 Users or 100 concurrently using clients?

They can be normal hardware with 16 GB Win2012R2 running IIS if you need to run FA on the host itself.

PHP and Apache don't have official supported 64 bit Windows builds.
Only MySQL out of the three does, hence no 64 bit bundle for XAMPP on Windows.

However, the current versions of PHP seem to have some 64 bit builds at https://windows.php.net/.

Processors, manufactured after 2011, most likely support AVX and it's PHP builds. The exact information about the hardware can be retrieved by using the Coreinfo tool.

Current Apache for Windows 64 bit are available at Apache Lounge.

In spite of all the above, give the 32 bit version of XAMPP a try on Win 10 with the specs in my previous post and provide benchmarks on actual concurrent usage and load factors in your network.

Yes, It sports the following in WAMP 3.0.6:

Apache 2.4.27
PHP 5.6.31, 7.0.23, 7.1.9
MySQL 5.7.19
MariaDB 10.2.8
PhpMyAdmin 4.7.4 
Adminer 4.3.1
PhpSysInfo 3.2.7

Wampserver 2.5 onwards are not compatible with

Windows XP, not even with SP3
Windows Server 2003

and they also require a disk formatted as an NTFS partition, and does not support FAT32 or exFAT.

Caveats:
If you install Wampserver over an existing version, not only it will not work, but you risk losing your existing databases. Also install it in a root folder like C:\wamp or d:\wamp . Close Skype or force not to use port 80 before installation and while WAMP is in use.

WAMP 3.1.4 was released on 2018-09-25.

Thanks all for your input. I will post my feedback once done