Engine

ERRRRRRRR NO!

OM606 is very indepth

OM605 not much easier although it is and probably more suited to the SWB

OM602/OM662 also worth considering

Assume it is starting as a 230 M102 with manual Getrag 5 speed then box could be used

230 M102 is left side starter like all the OM60X 

280 M110 300 OM617 are both right side starter so box change required

Even putting a 602/605/662 onto that box will require moving the box and altering the prop as your putting an extra cylinder in of similar capacity

You don't say what was in the car originally?

Gav knows... Long years ago our SWB had a 606 diesel engine (with a pugged up hole in the sump) and an automatic gearbox (with a non-pugged up hole) bodged in by a bunch of incompetent rascals (cf. P.N.) somewhere near Belfast. Once we'd invested in the leaky stinkbox - with potential - things were tidied up near Blackpool and a 6 speed manual gearbox from an SLK replaced the comatose autobox. Fast forward to 2023 and a legendary character not far from Bristle has sorted out all the mechanicals and we are (well, we always were, but now we speak from happy experience) firm advocates of a properly set up 606. As Gav mentions, it is neither a simple five minute nor a cheap job. However, it is worth it. (ioho)