Do I get the vendor to address any boundary issues?
Yes but unless it all ends up in the contract duly verified and signed off by solicitors/engineers........
Do I get my solicitor and/or surveyor to investigate further?
If you are serious, yes. Solicitor can ask for copy details of any agreements/wayleaves/disputes etc.,etc. He can then pass that on to surveyor.
How can I do this without incurring solicitor/surveyor fees if I decide to walk away?
You don't. You either let the vendor try to satisfy you now ( and to be honest, if I was the vendor, I would have addressed these issues before putting the property on the market. ) and then go to the solicitor/surveyor or you to go to solicitor/surveyor now and try to work out what you need to get from the vendor to explain the issues.
Some people are just messy individuals and leave messes like this unaddressed for as long as possible/forever.
mf