I started at the ISXKB, a huge resource on Inno Setup knowledge and sample scripts, but the holy grail wasn't there. Nearest is this page though, and there are also a couple of links to external script sources. This post will be added there, I guess...
As there's no suitable, ready-made, working sample available, I started with the (nicely modular) scripts of stfx @ CodeProject. There are quite a few updates and improvements hidden in the comments at the end of the page, that I missed at first ofcourse, so I had to update my changes again. That's one of the reasons we will never get out of work in IT, changes, changes...
The zipped-up results can be
+ (module) .NET 4.0 download & install for either/both Client and Full profile (using the web-installer, so no Language-pack hassle, it automagically gets the right localizations)
+ (module) VC++ 2010 runtime, downloading and installing both the x86 and x64 redistributables on x64 OS's
+ (module) SQL Server 2008 R2 Express for either x86 or x64 OS's. If SQL Server 2005 Express is already installed, nothing happens, that needs a manual upgrade or parallel install
* (update) SQL Server 2005 SP3, as the original extra module (suggested in the comments) couldn't install the RTM package on Vista or later
* (update) Restructure of the example setup.iss, so a single #define enables a feature
! (bugfix) Blocking the install if a non-privileged user runs the setup without using 'Run as Administrator', and one of the prerequisites needs to be installed. This enables PrivilegesRequired=none capable application-installations if all prerequisites are met.
* (feature) Rebooting at the end of the install if installation of a prerequisite requests a delayed reboot (MSI returncode 3010)
Any comments are welcome.