First, I am aware of paper wallets and I don´t want to use one for this purpose.
I will have to manage some bitcoins wich don´t belong to me in near future and so I need something with maximum security.
As long as you have the ability to spend it with the correct procedure, it is about as safe as a hardware wallet.
I have looked in a few solutions but
https://bitkey.io/ seems most promising.
Has anyone experinece with bitkey or can recommend something else ?
Never have I seen bitkey before, nor can I recommend them as your first option. I would recommend either Trezor or Ledger. They are quite well known and their source code has been vetted quite well, do consider installing the OS that the wallet is running on yourself.
If you want something which can provide similar security for a lot less money, I would recommend getting a Raspberry Pi and installing either Armory[1] or Electrum[2] on the Raspberry Pi. The steps are in the link. For Electrum, I would recommend using Tails OS instead of running it on Raspbian. Tails has it preinstalled.
[1]
https://www.bitcoinarmory.com/tutorials/armory-advanced-features/offline-wallets/[2]
http://docs.electrum.org/en/latest/coldstorage.html