Louise,
It's difficult to offer much clarity in an estimate based sole on a post, but I'm assuming you know that already give how thoughtful and deliberate your post is...and you're pretty much on the money, what you list in your submission is the MINIMUM. There's a pretty good discussion over at the Shero Designs Blog. I tend to agree with the broad statement that given the costs of licensing Magento EE and the costs needed to implement it properly, you should proabably start thinking about it if you already do (or plan to in the next 12 months) between $5 - $10 million per year in online sales.
The upfront and recurring fees are not inconsequential, but they do come with Enterprise Support which can help provide a consistent level of business support and remove the burden of mainintaining and operating the site from your team. Keep in mind that unless you go for some of the higher-tier Managed Hosting Plans, the costs you will see from hosting providers usally cover hosting expenses ONLY and support relating to those specific services. Not all do that, but many do, so be thorough and do your due dilligence to make sure you are comparing apples to apples when you get down to reviewing RFP's between service providers.
You can usually engage either your hosting firm or your consulting firm / agency for additional ongoing suppor to either supplement your onsite team or provide the technical resources. I normally suggest to clients that they consider owning the entire business side of ongoing operations and support (which may start with catalog management, marketing pricing, etc...), and take whatever budget they can afford and staff someone as skilled in house as well as working with a vendor. At some point, if you want to be successful, you're going to need to have people inside yor business take ownership of the project.
I've worked with some fantastic technical and business partners on Magento projects, but at the end of the day no one else CAN take ownership of your project and drive its success like someone with a personal & professional stake in your business. If you budget is constrained at first, then focus on the business side of things first and make sure you have room in your next budgetary cycle for continuing to invest in internal resource development.
Every customer is different, but normally I see clients go one of two routes: re-platforming every 3-5 years or executing smaller projects each year to maintain patches, security fixes, and add capability upgrades as new versions are Released. Which option you choose should be based on your business goals, but I mention it because may people think that the biggest expense they will have is on the original launch. In practice (and this is broadly true about all sofware from almost all vendors) you wind up spending 1/5 - 1/8 of the Total Cost of Ownership on you launch an the rest on the upgrades, maintenance, operations, updates until you replatform.
You may wind up no the high or low ends of that spectrun depending on your specific needs, but don't be so focused on the immediate launch project effort that you overlook that you're going to be using this system everyday to generate genuine value and (hopefully) profits for the buseinsses and you're going to need to plan on investing in that effort for several years if you want to ensure the highest ROI's.
You may want to consider checking out some of the Magento Resources compiled on github by Alessandro Ronchi. I've also shared my notebook of Magento Project Management Resources via Evernote, I hope they help!
- Bryan "BJ" Hoffpauir : CTO @ Comit Developers : https://comitdevelopers.com
For immediate needs I have a test # hours retainer with a developer who is/has experienced developers on the team and arranging hosting separately. I guess that means then that opens it up to other possibilites in future.