Hi ! I want to first create a static website with Wordpress using its static pages feature and then maybe if the site will grow enough to upgrade it to an online portal... Can Wordpress sustain this or is just well suited for blogging?
It's not the software you should question but the resources you have such as internet speed, RAM and CPU.
in anyway, as long as you have the "raw" contents,(database) of your website, its easy to manipulate, start from scratch, cms etc.
Go with WP, I don't know any better solution. If your site grow then you have great wordpress plugins for caching.
Yes It can be used as static website. It's very easy to work look one of the websites I created. malaysianagro.com
wordpress is very smart site. When your site will grow then you have to by that domain from wordpress and after buying it you can design as you wish.
Since you are asking the question.. I'm guessing you might have a lot of plans and you're right, you don't want to face growin pains later based on the web software you choose to utilize. Wordpress is great and has a growing CMS capability as well.. I'd project every and all features that you might consider some day and then highlight the primary and most difficult 'portal' or CMS features to use for choosing your CMS.
There are many non-blog sites powered by WordPress . In example, have recently noticed that http://www.couponsitescript.com/ is based on WordPress and is a script homepage / online store and presentation website. There are many plugins for WordPress that allow you to build virtually all types of sites. You can even build a community / social network with BuddyPress.