As Sergey has already told you, you need to come up with your own ideas and your own wit should be used in order to proceed. I can't force you to use a Website, I prefer Web Pages for myself, Sergey or some other might ask you to use MVC because it has a better object model. These are known as "opinion based" questions.
Somehow, I would leave the question "web application or web site" but will answer the remaining questions or yours.
A portal page for redirecting users to different applications.
A simple
Response.Redirect("~/resoursce.url"); doesn't fit your needs? I think, a simple redirect to the other website would be enough. Since you're using the Server-side code CORS won't be applied and the user would be redirected successfully.
http://msdn.microsoft.com/en-us/library/ms524309(v=vs.90).aspx[
^]
It will be implemented on an internal server inside a company.
Not a big deal, your web site as well as web project just needs a storage location where it can store its resource files for the clients to download from. Just add a static IP to your system and let the users download the files from your computer. No need for any hosting service.
Might giving this article a little read though,
Build Your Own Web Server[
^]
It have to be compatible with IE 8.0 or any other higher browser.
This is a client-side related issue and is more like to be taken care of by the JavaScript or the CSS engineer (well, developer I must write). IE8 is never a headache for any ASP.NET code. You need to take care of the jQuery (JavaScript) or the CSS code that you write to make the website friendly with IE8.
It will be integrated with two Databases SQL server DB and Oracle DB.
I would still prefer using only one Database, but using them both isn't any issue just make sure you're able to find a good Visual C# library for the Oracle Database, SQL Server won't be a headache.
I need monitoring and administrating privileges of the user on it.
Build a CMS! Buy a CMS and add it to your website. Many, many, manyyyy options for this. You can also try out Google WebMaster, Google Analytics or other tools which can handle this job for you. But, watch out if they say they're free!
High performance is required also.
A tip I might give you, "write lesser code". This would make your website a little bit faster, messy code causes the processor to perform lengthy tasks. Use caching, cookies etc to make sure you don't have to download the same content every time.
Caching in ASP.NET can be learnt about from
MSDN[
^].
Good luck.
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