Thank you everybody for all the information, much welcome!
Some background: The system has been running successfully for a number of years already;
only now the lead-acid batteries have deteriorated so much, that the refrigerator can be run only in daytime.
Therefore, i am going to replace them with LiFePo4 batteries.
On this occasion, i was thinking of buying a new charger/inverter too.
After studying the tech-specs yesterday, i have seen that the minimum amount of power required for the C/I combo recommended by BritManToo (which looks like a decent machine) is 6 panels connected serially.
As i would like to keep my current setup of 4panels parallel, i may stay with the old 24V 1000W Inverter and only invest in a new charger. (most likely PowMr, thanks to Crossy).
Does anyone have recommendations for a reliable source of LiFePo4 batteries here in Chiang Mai?