Don't do it. If they know what they're doing, they can give a quote without getting into the code. AlHefner and IncredibleHelp are right on the dime.
The
SEO company should be knowledgeable enough to figure out how much work is needed if they know the software brand that generates your pages (name of third party software) and they can research it from there.
If they don't know the software, then they should be able to come up to speed right away with learning how to optimize your pages. If it's custom software, you don't want it jeopardized.
Besides, as I understand it from reading this forum, what the user sees is what's important. And that is usually a simple interface that's used.
As a last resort, make another server account - read only - for them. And monitor their activities.
Accidental deletes do happen.
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SEO specialists, then make your decision. This company can't be the only one that can do the job.