Firstly: Bank account
You can open a bank account without having a house in Spain - no problem there. Just go to a bank with your passport and proof of address (i.e. bank statement) and that's it - account open in about 1/2 hour - easier than the UK.

Second: NIE
You can't get one without a Spanish address BUT you can get one as soon as you have a pre-contract (even if you haven't signed it) as you then have an official address (make sure the contract is in Spanish, and the 'flavour' of where you are - i.e. Catalan in Catalunya. The Spanish don't like issuing an NIE number to a 'holding' address any more. When you apply you MUST go in person to the police station with your completed application form and passport. They will peer at you and your passport. They will then stamp your form and give you a date when you (or someone on your behalf) can collect your form. In our case it was about 3 weeks but it varies from place to place. When you have this you can buy your house.