I can't say there is much difference in handling between the 300 and 400 series. The DR253 is a bit more pnderous but still a delight to fly. The lower sill on the 400 series is nicer and the sliding canopy makes getting in and out a lot easier than the bat doors.
Pretty sure you can re-engine the DRs (now even with a Thielert) but what you have to bear in mind is that most of the smaller engined ones will only have the main tank in the fuselage which is not so much of an issue with the lower fuel burn associated with the smaller engines but more so if you start using 30-35 lts per hour with the 160/180s.
The smaller ones also do not have a bagage door.
In the cruise there is very little between the smallest (108) and biggest (180) engine, the main difference is the acceleration on grass, climb performance and load capacity.
Interestingly enough the DR500 with the 200hp engine and VP prop does not perform any better than the 400 with the 180 fixed pitch prop. The 500 is a few inches wider though.
Everyone fallen asleep yet?
FD