I would strongly recommend you use either MySQL or Postgres as a database, and Apache as the server. Not only are they free, they are industry standard tools in wide use and the best tested platform for Moodle.
Many institutions, including my own, choose these solutions for Moodle even though they have licences for other software (Oracle, and probably various MSFT things, in our case). Remember that a) these licence payments are sunk costs and should therefore be ignored in any cost-benefit analysis, and b) most licences are for a limited term, so setting up Moodle to use them will create switching costs that may lock you in (i.e. cost you money) at a later date. It's probably worth checking what your exact licence terms are, often they are restricted in some way e.g. number of machines or processors, number of connected users etc.
It's quite unusual to complain about the speed of MySQL, usually people complain about all the advanced (and sometimes not-so-advanced) database features that MySQL left out in favour of simple, fast code. If you know roughly how many users you'll need to support then perhaps others supporting similar loads could ease you mind about performance.