QUOTE: “Think about whether or not this is a link that would be on your site if it weren’t for your actions…When it comes to guest blogging it’s a situation where you are placing links on other people’s sites together with this content, so that’s something I kind of shy away from purely from a link building point of view. It can make sense to guest blog on other people’s sites to drive some traffic to your site… but you should use a nofollow.” John Mueller, Google 2013
Ask for a technical and search audit for your site to learn what they think needs to be done, why, and what the expected outcome should be. You'll probably have to pay for this. You will probably have to give them read-only access to your site on Search Console. (At this stage, don't grant them write access.) Your prospective SEO should be able to give you realistic estimates of improvement, and an estimate of the work involved. If they guarantee you that their changes will give you first place in search results, find someone else.