ZKD at 282000 km defective – worth replacing engine???

After sensor change, timing belt change and wheel bearing and climate compressor exchange as well as new tyres in the rear now the(!): Coolant loss after a short ride. Even after approx. 15 km driving distance the coolant tank is empty! Display in the on-board computer: Coolant level! Then I have already filled up about 2.5 l water already 3 times after each ride Scrap or replace engine installed? I think to repair the head seal (workshop) is not worthwhile at the running performance. But is a replacement motor with 116000 km running power from 1998 for 600 € worth? Oh so: For the exchange I need workshop help – I don’t trust it – ‘(again 700 €) and the next technical test is in September – I am confident , but you never know before what the “ING” finds everything. Thanks for all the hints.