Insist, in advance, that when you come to Zurich, you will want to meet with the tenant from whom you will be renting.
He must, by law, get written permission from his landlord, to sublet. Ask to see this permission. (You are not strictly entitled to it, since it is between him and his landlord, but it does lend a certain security to see it). You could also, once you have the contract in front of you, say you want to add the clause that says that this agreement is based on the permission given by the landlord.
Do not sign any contract you do not understand.
Do not sign any contract you do not understand.
Do not sign any contract you do not understand.
If you anticipate that the contract will be in a language you do not understand, then take your own translator along. Or, better still, take the contract away with his signature on it, tell him you need to take it to the translator, make sure she explains everything to you, and the return the contract with your signature the next day.
Once you have met the tenant, viewed the flat, read and understood the contract, then go with the tenant (= your new landlord) straight over to a bank, and together (each with your passports) open the blocked Mieterkautionskonto bank account into which your deposit will be paid. Transfer the deposit into this account once it is open.... and nowhere else !
Because I suggest opening this account, in the branch, together, I recommend that you set the viewing appointment early enough to get to the bank during banking hours.
Make the deposit payment, and each subsequent rental payment, electronically, so that you always have a record of where you sent the money and that you did pay.