Yes, the PRAM battery is the usual suspect. I checked it already - 3.65V on board, that should do.

The iMac bongs and boots - but the monitor will only power up from cold start as mains have been disconnected beforehand.

Guess I´ll look for a used PC enclosure next summer on some farmers markets.

Already found a AOC TFT (for Win7 box) and a Canon LIDE (for Hackintosh) for 10€ each.