No problems at all ! Charge them by the hour. First get them to work out how many hours it will take them, and then hold them to that. If they go over, so be it, because the same would go if they overcharged - but that's up to you to ensure that doesn't happen.

Also get them to list some of their examples of previous work - and if you have any troubles in choosing, simply list them here with their proposed prices and time to complete, and we'll have ourselves a little vote.

Don't just go for the cheapest - that is some key advice!

__________________
The man who comes back through the Door in the Wall will never be quite the same as the man who went out.