The problem is not knowing whether potential housemates will be druggies/messy/noisy- I might not even get the chance to meet them beforehand if it's done through an estate agency
Absolutely, 100% DO NOT do a house share where you don't get to meet the people you're sharing with first.
I've been advocating for a houseshare upthread, but I didn't realise this is what you were considering.
You are putting yourself in a very vulnerable position if you move into a house with people you've never even met, they could be anyone.
Plus, think about it - they've never met you either. If you move into a house like that, the estate agent could move any old person in with you, you'd have no control over it.
That's nuts. Don't do it!
It's not the normal way people find houseshares. I've never done it through an agency so I don't know what's usually with an agent, but like another poster said, sign up with Spare Room, go meet them and see if you actually want to live with the people you meet.
Unless you are absolutely desperate to find a room asap, don't rush it.
Try to find a place you feel you want to live. Don't ignore any doubts . your instincts.