Users are hard to get hold of. I know there's ppc, seo, social marketing, but as a startup I want to drive large traffic. I know you can buy targeted traffic but I don't know what will convert well. My site will first target businesses looking for accountants and then it will add new services. I'm looking to expand this to a bidding platform once I get users
Totally agree with Andy's answer. But I just went to your site and I think you should really optimize your sign-up flow before spending any money or effort to acquire customers. Your current multi-step process and required information is far too cumbersome to efficiently acquire sign-ups. I would rework your sign-up process significantly before doing as Andy recommends.
Andy's advice is reasonable because you really want to test your channels and validate your core offering on a relatively small scale before looking to handle a lot of customers. LinkedIn is actually very effective for you as an advertising channel and though more expensive than other CPC's, it's far more targeted for your initial customer base.
Happy to talk you through what I mean about your sign-up flow in a quick call.