As others have already said, it depends greatly on your project - how will it be accessed? By who? How many? What will it do?
I'm a MS SQL 7, 2000, and 2005 DBA (I've pulled back from DBA duties at work over the last two years so haven't had a chance to play with 2008 yet) and MS implementation of SQL is just as fine as anyone else's but it's certainly not cheap (unless you compare it to Oracle). Although you can run a free version of MS SQL as pointed out already, it does have limitations. However, many great applications are out there running on this.
As for the DB accepting input from a variety of sources, that shouldn't matter. It's the client front-end that matters. Transact SQL is Transact SQL no matter what product is accepting an UPDATE or SELECT statement. On the other hand, your front-end may have to accomodate a variety of possible configurations.
Is the client front-end going to be web based? If so, where will it be hosted? In the case of a web-app you don't need to think as much about the end-users as long as they are using a browser that you support. Or, is this something you are developing that other people or companies will install and run on their own systems? If so you want to chose something inexpensive or something they may already own - or even better, something you can package with your product like MS SQL Express.
Anyhow, I gotta get back to work...
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