Don't quite get your story. Are you saying he already wanted a divorce and because of that, he engaged a property agent to sell this house. And it turns out that this property agent is the 3rd party to begin with?
Before you go to any lawyer, you need to be clear of the actions. Do you have kids and whats the terms like? You do know there is contested and uncontested divorce? Uncontested is way cheaper so you need to already know. But if you want to know more and your options, go to a few law firms to seek free consultation (it can range from 30-45mins). Usually they are not so strict abt the timing even though they might say its free 30mins.
Maybe try these PKWA, Eden Law Corp for some info first.
With all these, you need to compose yourself and talk to friends and family. Keep your sanity in check first before talking to the lawyer. Talking to many law firm might drain you too.
Stay strong and know your options.