You might want to look at other options for data storage then, as well, provided that DH already told you that you will not be able to handle that amount of storage and traffic on any of their accounts.
Amazon S3 is geared towards big scales (I have seen reports of people using dozens and hundreds of tbs of space there at $150/tb/month for pure storage plus traffic). Provided you can chunk up your content into pieces smaller than 5 gb, that might be useful to you. Personally I haven't used them for things other than the occasional site backup, but it should be possible to use it as an offsite data storage provider in most webapps, easily (the API is geared towards that, though you can also use it for plain static file distribution as a CDN).
As you don't say what kind of access to the data you expect, it might even be possible to use the DH dedicated servers for your data processing needs and fall back on S3 when you need data (if your data is mostly "sleeping" and not constantly accessed, the bandwidth fees might be negligible; If your content is sufficiently static to be served to your site viewers from there directly, you don't even need to "proxy" it through DH). If not, looking at Amazon EC2 to get some processing power as well might be an option).
As for 1-2 million unique visitors a month -- Depending on the type of traffic even an old celeron 1.3 can handle that kind of volume -- though I suspect your application is not that type of traffic
In any case, good luck