over 15,000 users currently use the desktop version of this application; we expect about 80% of these would log in and use a web-based version in single four-hour timeframe.

Looks to me like you need a UNIX based solution, based upon the number of users over time.

We have other web stuff using Sun servers

Aha.. so you have some Sun experiance already - If the client has the cash, run Solaris (rock solid)

One other factor is that whatever solution I choose, the data will be stored in an Oracle database.

Oracle and Solaris play very well together.

My questions are:
1. How closely can I make a web version look to my desktop version with .net (assuming that it is a straight-forward windows app)?

You should probably be asking "how can I reproduce the functionality of the desktop app" rather than trying to make the look-and-feel the same. Web development is a different beast than desktop development, and not everything that works well on a desktop app will transfer to the web easily.

You haven't really spoken to the complexity of the application. Is it just a reporting tool, or does it do much more than just getting data from the database and displaying it? If it's just a reporting tool, or data query tool, it could easily be done with many different web development tools on Solaris (sorry, I don't consider win32 a viable server platform)

PHP with Apache/SunONE - easy to write in, not very modular

Java with Weblogic or JBoss - write your apps in beans, then they are modular, and Weblogic or JBoss will provide you with load balancing, etc (they are containers). There are a lot of tools to use for Java development (NetBeans, Rational). Oracle provides you with some superb JDBC drivers that can be optimized for your load.

I'm a little biased towards Java development because I've seen it work for me in enterprise apps before. I've used PHP some (I wrote a web-based e-mail package in it) and it just doesn't strike me as a viable enterprise level platform. Java was designed that way, and you can get a lot of support from other companies for Java that you can't get for PHP.