Mine is petrol and works like a charm. There's a guy in Poland who sells used ones on ebay, could have been I found him on German Ebay. It came with a new wiring loom, new timer and loads of coolant hose, fuel lines and connectors. Complete package for about £500. I added a GSM relay to mine, which is sweet!
I don't think T'ing into the coolant lines will work very well, the problem being you will have no control over the flow of hot water. You could end up circulating it only through your matrix, leaving your engine just as cold. If you can plumb a hose from your tee to the heater, I don't see why plumbing it in series - which is the proper way - would be any harder? If you don't want to waste any heat in the heater matrix, simply plumb the webasto in downstream of the matrix instead of upstream. This way your coolant will transfer pretty much all its heat to the engine before it even reaches the matrix. This is not so good, though, if you plan to hook up the fans later.
LOL
Henrik