Is it possible to take that private key and import it in a trezor (or any other wallet)?
Hardware wallets do not support importing private keys due to security concerns. You should be able to import it in other software wallets. You can send your coins without any problems to the Trezor T once you buy it.
I was thinking of getting a model T and a model ONE as a backup. Is it only to take the backup seed and restore it in the model ONE? But model ONE doesn't support everything that model T does, will that work?
You can restore the wallet using the seed in every wallet which supports BIP39. You can restore the seed generated by Trezor T on Trezor One and vice-versa. You can also use Electrum if you lose both wallets.
Is there any other disadvances that I should think of before buy a it compared with other hardware wallets?
You should take a look at
this vulnerability. You can mitigate it by using a long passphrase. Unlike Ledger, Trezor does not generate native SegWit addresses (the ones starting with bc1) in the official web interface. You will have to pair your wallet with Electrum to use them. Ledger is mostly focused on adding altcoins while Trezor has just implemented
Shamir Backup. Please note that it's only supported by Trezor T. Trezor has a much better implementation of multisig than Ledger (you will see all inputs and outputs properly).
I have both Trezor One and Trezor T so I can answer any of your questions.