Starting glad that you see it isn't about the OW at all.
Look at how little he did in your relationship and how much he let you do for him. He is just doing the same with another willing victim.
The text may have come from her however he is the one behind it.
He will not have told her it is your house and he has no claim but that he is being generous as he feels guilty leaving you and letting you have it.
He will not have told her that for the last 15 years he has sat on his arse and let you pay for everything. He will have told her that he was equal or more important part of your relationship doing everything for which you do nothing but show your ingratitude.
He certainly will not have told her that he came and asked you for money the week after he left her for his DDs. What a man 
He will not have told her you paid for the van.
He will not have told her you were doing all the behind the scenes work to get his business off the ground.
He will not tell her all your savings have gone on him, his van and his business.
He will have told her he is an amazing man who has done it all by the sweat of his own brow.
This man has never been your friend. He has been (great mumsnet term) a complete and utter cocklodger. Now that he has taken everything decent you had to give he is off.
Mark any post RTS and stick it back in the letter box. He does not live with you any more it is none of your cares what happens to it.
Send him a solicitors letter instructing him to remove his stuff by x date or it will be disposed of. The van would not be tit for tat. I have a feeling that you say that because it is how he will see it. It is getting something you paid for back. Get the solicitor to offer to sell it to him for a fair or just below market rate if that would make you feel better.
Get a new mobile number. Only stick the old sim in once a week/month if you feel you have to in order to check on communications from this man. The same with your email. Or get him blocked.
This last text in response to bills he would have partially created is offensive. Cut him out of your life as soon as you can and move on.
You are worth so much more than this man.