Is bitcoin-qt as PPA really secure? How to proof? With Bitcoins being that much worth currently, I came to the conclusion of using a unique old computer just for this "use case".

I would like to run Ubuntu with bitcoin-qt (from PPA), but now the paranoia kicks in: what if the client is corrupted? I know that I can check against the MD5/SHsomething, but who guarantees that the client itself is not corrupted due to an error/mistake or even intentionally?

It is said that with PPA, nobody guaratees that the software itself is not corrupted, so I am unsure about transfering quite a lot of money to some software I don't know anything about.
You can instead directly install from the source code. It is available on the project site along with release signatures.