What is the difference between the 100bhp and 115bhp Tdci engines? is it only the ECU?
This is something im looking at at work with a mate everything looks the same. it does seem to be all in the ecu

Can the 100bhp version be upgraded to 115 by simply replacing the ECU?
not as simple you need to take the 115 programming and put it on the original 100 ecu #

Which model has rear disc brakes?
the 115 bhp one would but it maye depend on trim spec

The 1.8Tdci engine is belt driven and 8 valves not 16 right?
right
Any comments on this engine? What should I look out for?

How does the Tdci compare to the Tddi? Which is better?
tdci is better. tddi is mechanical fuel distrobution whereas tdci is common rail electronic injector control