Let me elaborate on Help Elf's official answer a bit, albeit from my personal perspectives both as an ex-SBIer, and as someone who now helps clients build, manage and monetize their websites.
My question here is...how do I know if the company that built my site and is hosting it now can continue to host if and when I get the SBI plug in? Then, how much should that cost?
If you get SBI for WP, you'll just install the plugin on your current WordPress site (just as you install any other plugin). The plugin will connect to SiteSell's servers and do the magic in the background. So, no, this will not impair your current provider's ability to host and manage your site in any way.
Note that what you're buying is not the plugin, but the service. This plugin is only the bridge that connects your site to the SBI for WP service. This is similar to how you would install a WordPress plugin to connect to and use services like MailChimp, PayPal, Google Analytics, Facebook, Twitter, etc. The SBI for WP service costs $149/year or $17/month.What you're getting are: the incredibly powerful keyword research tool, the step-by-step Action Guide, and the awesome community here.
How useful would these be to you? Only you can decide.
But, in my opinion, SBI (and SBI for WP) is best suited for the DO-IT-YOURSELFer. If you're not actively involved in the building and running of your site, neither SBI nor SBI for WP will be that useful for you -- unless your "local search specialist" is deeply rooted in the SBI philosophy and tools; or is willing to adopt, learn, and fully leverage SBI tools and processes.
You could use the tool and guide to research and plan the viability of your business (site) idea, your monetization strategy, and content blueprint.
Or you could hand it off to your provider to munch on it.
The chances of the latter happening, in my opinion, is next to nill though -- unless your consultant is a loner who is barely scraping by and would do anything to pay the bills.
Putting on my provider hat now, I know from first-hand experience how incredibly inefficient (and expensive!) it is to adopt every tool that every client brings along. So most established consultants standardize on their own collection of tools and only support these, or charge a hefty premium to support client-provided tools.
If you go with SBI, then your entire site (and domain) will have to be moved to the SBI platform, and the site re-built from scratch.In closing, wither with SBI or SBI for WordPress, first decide how much you want to be involved in the on-going development and running of your site, and have a conversation with your current consultant about your choice of SBI or SBI for WP.
Finally, note that there's an independent, but officially-endorsed network of SBI consultants here. If you're absolutely sold on the value of SBI or SBI for WordPress, but your current provider is unable to help you carry the goods home, you could cut the cord completely and find new consultants here.
Just my two cents, as always.