Ulfi,
If the problem is indeed a rod-knock or any other wear related issue between bearings and piston/cylinder, and has been constant for as long as previous owner indicated, then it's strange that the sound (pinking or pinging) is not also occurring on any other cylinders.
If you need to pull and rebuild engine however, you might want to research the cost of a full rebuild (long block) by contacting Metric Motors (in Canoga Park near Los Angeles) and shipping costs. A full rebuild (long block) for a 250SL engine using my own block, cost USD $4.5k 4 years ago... but I see now that long block rebuild costs USD $6700 (230SL or 250SL engine) plus a "core deposit" of USD $1.5k in case one of your core pieces (block, head, &/or crankshaft) has to be replaced. Shipping charges (both directions) from my CA location cost me nearly nothing though. I would guess that shipping (slow boat) your block from Denmark to LA and back would be perhaps $1k - $1.5k... so total cost would probably come to something around $8200 USD if none of your core items needed to be replaced.... that wouldn't include cost of removal/reinstalling the engine though.
If you're forced into rebuild and you have to pay USD $10k to have this done in Denmark you may want to consider having the premier MB engine rebuild shop do the work.... total cost may even be less than $10k.
I still have to suspect a problem unrelated to wear since only one cylinder has the sound (pinging)... also pinking or pinging sound is higher pitch than the "knock" sound of a rod-knock... decidedly different in tone. I agree with Shvegel however in-as-much-as the possibility of a rod knock, bearing wear, or piston slap potential does exist. An experienced mechanic can, on the other hand, discern the difference between sound due to wear (knock, slap) and pinging due to detonation / combustion problem.... especially with some diagnostic tools at their disposal... stethoscope listening predominantly. As an engineer, a frequency analyzer tuned to various audio ranges can discern the differences in pitch... but I'm not sure any normal mechanical shops would have the equipment (audio frequency analyzers or techs that can use them) so that might not be of any practical value to you.
I suggest (still) that you retard and advance timing sufficiently in order to see if the pinking or pinging sound dissappears (with spark at Cyl #5) without also starting to occur on one or more of the other cylinders, but with engine still able to run of course... at higher rpm and during acceleration of engine rpm. If advancing or retarding the spark causes Cyl #5 to stop pinging without the other cylinders also starting to ping, then it's not a wear problem. If on the other hand a small change in advance or retarding the spark causes the other cylinders to start pinging as well, and IF the sound of pinging from the other cylinders due to advance or retarding the spark is higher pitch than Cyl #5's "pinging" sound, then you'll have to consider the wear problem issues as the dominant probabilities.
By the way, any of the wear problems get noticably worse with engine use, so if prior owner told you sound has "been there for years"... then he was being less than fully honest with you (sound should have gotten worse with engine use over the "years") or he didn't use the engine much for the "years" he used the car. Once a wear condition starts it snowballs... gets worse at a faster rate. A pinging sound that's "been there for years" isn't consistant with a wear-out of a bearing (rod, piston pin) or piston slap issue.
