Activate unsubsidized iPhone 5 - can I later switch carriers? I'm living in Germany and recieved an unused iPhone 5 A1429 without SIM or contract as a gift. I'm planning to use it abroad, likely with prepaid SIM cards.

The phone is not activated yet and I am very much afraid of it being locked to the first network or SIM card brand I stick in. Is that what happens during activation? What's the purpuse of activation?

Unfortunately I could not find any useful information from Apple, other than I have to have a SIM card inserted to activate the unit - WiFi won't do.
An authorized unlocked iPhone does not need a SIM card to activate the OS after a restore and it will work once you run through Apple's software setup screens when it asks things like if you want to set up iCloud or location services... Activation is just the process of checking whether the device should be carrier locked by querying a server of Apple's choice over WiFi and then updating the appropriate carrier settings if the ones loaded from the factory are out of date for a particular SIM or region.

Even in the US - iPhone 5 that are sold "under contract" to Verizon are not actually SIM locked and you can use them with any compatible nano SIM that supports at least one frequency and protocol that the phone model supports. It also is a A1429 model.

You can remove any SIM and power it on if you are worried about some carrier settings being locked.