You can compile a setup without a GUI at all. Then it will install directly when you launch the MSI I believe. Frankly I am not sure if this works properly with the UAC (elevation prompt), but I think it does.
As I answered in your other question, you can also use a merge module to allow your setup content to be merged into another setup at build / compile time instead of running as a setup in its own right.
If you want an MSI, I don't think it matters that much if you do not localize the setup to all kinds of languages. Just link a default English GUI. You can even include ready-made translations for several languages as far as I know (been ages since I have done real localization).
The real point is that your customer can kick off your install in silent mode easily from within their own setup.exe, so your setup GUI will never be shown - just make sure to document this for your customers so they know how to do it.