Changing to a dual row change has become quite expensive. If you can find an engine, which is in decent shape, which already has it, it will save you a whole lot in both parts and labor.
It is not imperative to convert to dual row chain if you've got a single row... it would be a good idea but is not absolutely necessary. The single row chain can be left in place as long as the oil is changed at regular intervals and the chain, chain guides and oilers are checked and changed if necessary but I wouldn't go more than 5 years without at least checking them. It is a lot easier to install a new single row chain than it is to replace the engine.
I wouldn't go more than 10 years without changing the chain and guides if it is single row. It is so easy to check the chain and guides that there's no reason not to check them. Pull the right side rocker cover (passenger side in US) and take a look. The guides will be plainly visible (if they are dark brown change them immediately) and the chain can easily be checked for what people call stretch (it's actually wear) by pressing on the chain anyplace where it is not in contact with the cam sprocket (to take up any slack) and then grabbing the chain at the midpoint on the cam sprocket and see how much play it has by pulling up on it. If it can be pulled up from the sprocket by more than an 1/16th of an inch... change it.
Regular oil changes with quality oil are a must with these engines. You wouldn't believe how much oil is flowing in and around the cam sprocket, the cam lobes, the rockers, the hydraulic compensators and the cam towers unless you've seen it for yourself. I recently cut the top off of a spare rocker cover and bolted it to the drivers side so I could watch for myself. The oil flows like a river up there, it drops directly down on the cam lobes from the oilers above. The oil gets to the oil tubes from the tops of the cam towers and oil is fed to the compensators via holes in the head. No wonder these cars hold almost 10 quarts of oil... a good portion of it is circulating in and around the top end. I couldn't let the engine run for more than a few seconds before oil was all over the place.
Oil filter changes are a must at every oil change... don't go by the old adage that says you can change the filter on every other oil change... it's few bucks and a few minutes of time that are well worth the effort. Maximum lubrication of the top end, which includes the cam sprockets and the chain, is essential to the life of the chain and everything else up there. There's a reason Mercedes designed the oil capacity to be almost 10 quarts, it's best that we keep that oil as clean as possible.
Sorry for going on and on... in the past I abused my engine by not changing the oil frequently and I paid the price with a blown engine twice due to the failure of the timing chain. My ignorance about the importance of frequent oil changes and checking the guides and oilers cost me many thousands of dollars... my hope is to save others from that same fate.