I will try not to be too biased as I am a huge PHP supporter, especially over ASP. There are many sites and discussions on "the best" web scripting language... do a Google search on "PHP vs ASP" and you'll see
A few things I can think of is PHP is more portable, it can run on various systems and operating systems, as opposed to ASP which is the work of Microsoft. You won't see ASP running (happily) on linux, confining your ASP solution to a Windows-only platform which may raise issues with your concern for security. PHP is also completely free whereas I think ASP requires licensing? On the other hand, many will show disfavor for PHP as it is sometimes deemed "insecure"
My overall point is you can reach your goal with PHP or ASP, and make it fast, efficient, and secure - so I would recommend you stick with the language you know best - security issues are usually introduced by developer ignorance, so whichever you choose - read up on some best practices to writing secure applications... for instance with PHP, check out php|architect, its a great magazine
As for the database server, I don't see why MySQL wouldn't be a great solution for you - it can handle your proposed workload. My only experience is with MySQL and Oracle (which would be overkill), so I can't comment on other RDBMS like MS SQL, etc.
Just my 0.05 euro