[steam for ever]

I do have a slight soft spot for both 153 and 156 even though my username does not suggest it.
They are very comfortable trains indeed and uselful ones too.
I take a 156 part of the way to work each morning and the trip is very enjoyable.
The 153s were rebuilt from two coach 150s which are also nice to ride on and Hornby produce models of those.

The self contained nature of DMUs is very useful so may I suggest this.
If you want to build a kit there are several steam rail cars on sale which are good for branch lines.
What ever floats your boat really and remember its your choice.
Have you ever ridden a steam train?
It is easier to add more coaches with loco hauled trains though.