I run a small company, and own a hyundai H200 (france) there is a knock in the engine when above 1500 rpm (done 240,000km i went to the official dealer in Bergerac to have a look, charged 500 euro, and tells me if there is anything inside the engine wrong then they do not repair but replace the engine, asking how much it will cost, i was given a quote of 12500 euro it is a 1999 car. while i was happy with the brand, this has put me off hyundai.

take it to the parts store and see if they can erase the code and reset the computer for you.. it may work..

Most likely the engine harmonic balancer on the front of the crankshaft is coming apart. This is a fairly common issue.

Have a oil pressure test done on the engine to determine bearing failure. Have a compression test done to check for internal damage such as broken rings/pistons or overheated motor.. If the noise appears during acceleration then run the fault codes and look for a tuning problem or failed knock sensors

Do a leak-down test or at least a compression test

It could be that your knock sensor circuit is working perfectly.You may have a knock that it is reading for long periods.
Try running it disconnected, and see if there is a clatter or knock when accelerating lightly.

You could have a vacuum leak.

could be a vacume leak. check hose for cracks or lose conections.

Look for a vacuum leak (lines and intake), Is the engine light on, were any codes present? A faulty EGR or IAC could cause the problem too. Check the throttle cable for any restrictions.

either you need a new tranny or your ecu is reading a code. Same thing happened to mine, i had my transmission replaced.

Have the alternator tested. You are providing a low charge to the battery when you jump it and when taking it above 2K RPM requires more amperage resulting in the engine dying and the rough idle

First, nothing you have done has anything to do with valve tappet noise. What is your oil pressure? Have you positively identified the noise as a lifter (tappet), OR COULD IT BE AN ENGINE BERARING OR WRISTPIN? Other than tearing it down and replacing lifters, rockers or pushrods or possibly even the cam, if oil pressure is good, change the oil and put a quart of Marvel oil in there. If that does not clear it up in a few days, you will have to go inside the engine. You likely have over 160k on that engine...consider using a replacement. Get an older tech to listen to it...it takes years to be able to differentiate between different component noises with any accuracy.
Personally, without hearing it myself, I'm betting it's a rod knock.