I make hundreds of transactions per month with MEW and you do not need the JSON or password file. Only the Private key is necessary to access your funds. Keep this private, safe and backed up.
Password and JSON files is one way of accessing you account or Myetherwallet address. In fact it is safer to use this method than accessing your account via private key. On the hierarchy of option to access an etherwallet address, private keys are the last option as this is not recommended way.