Party wall agreement

Planning permission and party wall agreements are two different, although related, things. There is helpful information and examples of letters/agreements on the gov.uk website, including an explanatory booklet if you want to read 55 pages! I don't see why you need an expensive party wall survey, you should be able to handle the process with simple and clear written/email communication. No idea whether neighbours would oppose, but there is a process of 'dispute resolution' you can go through if they do.

if you have your planning permission then the neighbours cannot object to that. With the party wall you can do it yourself - see here https://www.gov.uk/guidance/party-wall-etc-act-1996-guidance I always advise getting a surveyor if you suspect there may be a contentious issue.