I need to create a web portal for a client's resellers where they can log-in, access information, watch videos, etc. Will Joomla enable me to do this, perhaps with the addition of some plug-ins, or is there some other solution, before I delve into it too deeply?

Not so long ago, we would have just had it written bespoke, but these days there seems to be an off-the-shelf solution for just about anything you can think of!

A hosted Sharepoint Solution or something like this comes to mind
http://www.dotnetnuke.com/

Cheers, I shall have a look at those, though I'd rather avoid windows server as it costs me more and I don't have any knowledge or experience of it!

I was thinking fully hosted solutions I must admit but if you prefer try this
http://phpnuke.org/

Well it would be hosted on my reseller space. There could be a budget to have it hosted and maintained by another company, which frankly, I'd prefer, but as I said, it comes down to budgets as ever.

It always come down to budget.
£6.75 per Gb per month
£15 per month .....

Not too heavy.

That's not bad. Is it easy enough to skin?

I'd try a few CMS packages locally before splashing out on anything. Presumably your reseller account is php / mysql and wont cost you anything to play with? If so, that's got phpnuke, Joomla and most others covered. If not, it's really quick to install a LAMP VM appliance on your local PC and test on that.

Unfortunately, most people who are into Joomla will tell you why it's the right solution for you. Most people who are into Sharepoint will tell you why it's right for you. It's a case of "if the only tool you have is a hammer, every job looks like a nail". Personally, I'm really into Notepad and I think you should code it yourself because it's far more efficient, even if it takes you a lifetime

Depends how much you want to change it. Plenty of free downloads and MS sharepoint designer tool is free ( I think ) . And I agree with JJ, we all support a tool we know.