

I’m not sure it’s ever legit for the job-hunter to be paying the recruiters. It would normally be the employer.
A % commission doesn’t give that much incentive to find you the very best job as opposed to the first one that will do. You’re paying them a percentage but they’re looking at the return per hour of work they put in. You’ll come under a lot of pressure to accept the first job on offer simply because that job gives them the best return even if it is a smaller cash amount than the best job they could possibly find (if they put the time in).
Their incentives do not align well with your incentives. So best avoided, IMO.
Cork insulation would usually be skimmed over with plaster. You could have a look at insulatiing plaster too, but I think that needs to be thicker than cork to work well. Less munchable by critters though.
In an old building, you need to use breathable insulation, breathable plaster, and breathable paint (and breathable mortar, if you’re repointing the outside). The moisture needs an escape route.