There is no need to engage solicitors until you have a buyer. No harm in getting quotes though. I have used solicitors recommended by the estate agent for convenience in the past and they were ok. Generally property transactions are relatively straightforward so I have not found there to be much difference in cost or service received between different firms.
My only recommendation is don't use the do it yourself online solicitors (Edit Premier Property Lawyers). They got the job done eventually but it required a lot of chasing and effort on our part.