I looked into this myself ages ago.
The six cylinder 190E (the 2.6) has a different 'slam panel' (the bit the bonnet latches onto), and thus different radiators. So you will need to change the front if you put a six into a 2.3. A have seen it done though and don't think it's a big deal. In an ideal world you would start out with a 2.6, as making that a 3.0 is really not a big deal.
There's discussion of it on
www.mercedes190.co.uk and a few members have done engine swaps there, including a 3.2-24v, 3.0 12v, and even changing a 2.0 to a 2.3-16.