Planning a large website 101
1) Start from the ground up plan your server setup and find someone to do the setup and run it long term. Your server guy will make or break your project. Make sure he understands VPS, GEO caching, clustering and most important security. Ex-hosting company level 2 seniors are great. Unless this is a mega project you should go for something that can scale with some easy so you don't need to migrate and possible cause issues while your in the rapid growth phase.
2) UI Design, you will need a specialist designer who can work closely with you and most importantly liasion with your Development team. Do not sign off on the Website designer before the Website Development team checks them out. The last thing you want to do is pay good money on designs and find out they are for an out dated framework. Making sure they have worked on Website mockup and in a team before is also important.
3) Developers, If it is a sizable project you'll need at least two possibly three developers or more. One will have to be the lead. You should have 1 backend developer and one frontend developer. Make sure your lead has managed a team before and worked on a project of a similar scale to completion!
4) Project manager: If your team is not from the same company or the company of developers is taking more of an oursourcing role you will need a project manager. Unless you are very experienced do not try and do this yourself. You want someone to be able to check things off, keep people honest and offer support and trouble shooting help if needed. If you are using one company for all or most of the roles make sure they have a project manager who will organise everything for you otherwise mess will follow.
5) Content writers, make sure the speak your language and know your product and market. What they write will be your companies portal to the world. You also want them to be able to keep the fluff writing down and stay on point with keywords.
6) Marketing: If the company you have hired has this covered thats fine. But don't be afraid to outsource this. It is a very specialised field. Make sure you go to the right place that understands the online business. Sending out press releases is all but useless. You want them to create a buzz online before you even get going.
Other resources you may will need:
Most important timeline planning. Make sure you plan everything out in hours. Don't plan to far a head with firm deadlines. If you are going down a fresh path stuff can happen and deadlines may need to be extended.