Is it obligatory to pay the full usurious debt (1700 Euros), or is it sufficient to pay the original amount (900 Euros) after repenting from borrowing from a usurious bank due to necessity?

You are obligated to pay 900 euros of the debt, which is the original loan amount without the usurious interest, because Allah Almighty says: "O you who have believed, fear Allah and give up what remains [due to you] of interest, if you should be believers. And if you do not, then be informed of a war [against you] from Allah and His Messenger." As for the lawyer's fees: your lawyer's fee is incumbent upon you. The fee of the other party's lawyer is not incumbent upon you unless the system does not allow them to sue you except through a lawyer. In this case, if you have proven your insolvency to them, then their fee is not incumbent upon you, because Allah Almighty says: "And if there is any debtor in difficulty, then [let there be] postponement until [a time of] ease. But if you give [it in charity] as a charity, it is better for you, if you only knew." If you have not proven your insolvency, then you are obligated to pay from their fee the amount by which insolvency is proven.