All the instructions will say that only qualified installers can install these units.
Ignore said instructions and you will find that even with pre filled units if they are not installed and commisioned by qualified engineers, the warranties are not valid.
Take the risk and do it yourself, or even get a general builder by all means, but if anything goes wrong and you try to get stuff fixed under warranty the first thing everyone asks for is the certificate of installation signed by qualified personnel.
Buy a decent make, and the warranty won't be needed, if you look after it.
I'm currently involved in the refurbishment of a 9 storey building - it has a chiller for the air con, which is dated 1964... kept maintained, AC units last a LONG time.