Don't use MS SQL server. It has serious issues with locking at scale that you can never get around. Apart from that, just make a choice and run with it.
I've worked with MS SQL for 10+ years (at financial institutions) with no problems. Oracle might be a tad better, but it's also pricier. No idea about Postgre (I've heard it's good), but what you mention shouldn't be a good reason to dismiss MS SQL.
The real _problem_ (which BizSpark can resolve short-term) seems like it would be licensing.