By this I mean that only the last x blocks are saved in the blockchain (x sufficient high to be sure there are no forks).
Let me stop you right there! The only reason to store and keep the whole blockchain is to be absolutely sure it's the longest chain since the genesis block. Without the genesis block, which is hardcoded in any full client, you suddenly have to trust third parties again!
Think about this for a while, as what you're suggesting goes against Bitcoin's fundamentals.
You are right, this makes it a bit more complicated, but I don't think this is a problem. The initial block of the truncated blockchain has to be downloaded from some other client. But then the client could wait again a few blocks to make sure that it is on the right blockchain, and not on some fork, or some created blockchain from the client from which it loaded the initial blockchain. Once it is running and new blocks are successfully added from multiple clients, it can be sure to be on the right blockchain (assuming it has some mean to determine that the blocks indeed came from multiple clients). It wouldn't make sense for other clients to send wrong initial blocks, unless they want to create a competing blockchain, and I don't see any way how this could be used maliciously.