The parts are relatively inexpensive, but its quite a job to exchange the chain.
If there is no way of defenitely finding out whether or not your chain has been changed, I would most defenitely recommend having it done sooner than later.
If the chain breaks the valves are no longer correcly timed and the pistons will hit and bend the valves, leaving the engine inoperable and making a complete head-rebuilt/valvejob necessary.
You should avoid any abrupt accelerations, thats usually what breaks the chain or stretches it.
The M103 straight six in your car is not really infamous for broken timing chains though, the V8's are though. But at your milage I'd defenitely have it done anyways. Better safe than sorry!
The tack , tack sound indicates that your timing chain tensioner is worn out and does not hold the pressure on the chain.
I would get the parts myself and have a reliable independent do the job. You'll need inner and outer rails, tensioner, clamping lever, chain and a masterlink. Collect the replaced parts though. You can never be too sure.
Good luck and happy motoring.
F.J.

__________________
F.J.
81 300 SD grey/palomino 168K miles
84 190E 2.3 black/grey 64K miles (wrecked)
85 190E 2.3 maroon/palomino 92K miles
88 300E desert red/palomino 204k miles
(made to look like a '94-95)
92 400E desert taupe/creme beige 120K miles
(converted to 94-95 E420 looks)