If you need reliability then there's a good chance that any reasonably good hosting company will be a better, more reliable solution than anything you can do yourself.
It all comes down to what you need to do however. Do you need to be able to access your DB information remotely i.e. from over the Internet or do you just need to access it from within the office? If the answer is just within the office then just use IIS 5.1/PHP/MySQL on a local PC in the office. IIS comes free with Win XP Pro but you have to be aware that XP Pro can only support 10 Concurrent Connections because it is it only designed as a development platform and not as a end user solution. IIS is a damn sight easier to use than Apache especially if you don't know what your doing.
With regards to people being able to access the network, again it comes down to whether you need to access it remotely. If you don't need access from outside the office then it is fairly easy to restrict access both in MySQL (see GRANT PRIVILEGES at
www.mysql.com) and within IIS to only certain IP addresses. If you know what your internal IP addresses are i.e. 192.168.0.* then you can limit access to only the IP addresses being used.
If you do need access remotely there could be many factors surrounding the security that would need looking into.
In short, personally I would summarize by saying that if it were me and I needed 24 hour almost guaranteed access then get an external host to do it for you. They're much more experience, invest thousands in Firewalls, data backup etc and if you do a bit of research and pick a good host they won't do a runner overnight so you don't have anything to worry about there. For what will probably amount to £200 per year you will have piece of mind.