What kind of computer should I buy to work with databases?
I need to buy a PC for an office that will work with databases of LibreOffice
This is a question that falls into the “How long is a piece of string?” category.
Given such limited information I would advise you first look at the system requirements for LibreOffice here
To give you a more definitive answer we would need to know: How big your database is likely to be, what would its performance requirements be, is your database going to be networked, have multiple users, simultaneous users, would the database access be the only process to will the machine have to support other processes, will the data be all key board input or batch, what kind of response to queries would you require, what are the requirements for reporting, etc. etc.
A simple answer would be ANY modern PC that can run LO is probably sufficient; but wether it would be optimal requires answers to the questions above (and probably others).