Are there any negatives to installing a mac-mini server as the core server for the purposes of storing data in a company using 5 PCs (HP-Micro Soft)?

Not enough data to answer the question.

For one, are there only 5 PCs in the company, or are there 5 PCs in the midst of 200 Macs?

Secondly, what do you want out of your server? 'core server' means nothing. What services do you want/expect to run? What services are your users used to/expect?

Are you looking for file serving? email? calendaring? directory services? DNS? DHCP? NAT? web service? something else?

Is this server supposed to augment an existing setup? or form the basis for a centralized infrastructure moving forwards?

Ultimately, a Mac server makes a poor choice for a Windows-centric user network, but without knowing more about your expectations it's impossible to say whether it's viable or practical.