Well I'm no expert!, but I've read of Mercs carying on for many thousands of miles without problem, just a tappetey-tap tap until the oil has completely warmed up. My top end is a bit noisy at the moment but I just dont gun the engine until it goes quiet after that its fine. Replacing the lifters is something that has been covered in many threads, as far as I can tell its not very hard proberly just needs time and a bit of confidence.
http://www.peachparts.com/shopforum/showthread.php?t=97866&highlight=replacing+lifters good thread and
http://www.peachparts.com/shopforum/showthread.php?t=85460&highlight=replacing+lifters you'll see a comment from Hassman who used synthetic oil to quieten his top end, I guess it just shows no-one really knows which type of oil works best!
Mike.