basically your tddi engine would be no good for anything, from the tdci range of duratorques, tddi pistons are different shape, your 155ps injectors are short nozzles not long ones like all tddi and tdci duratorque heads.
your tddi head also does not have a cam shaft position hole (allthough dont think it would be mutch hassle to drill the hole out were its supposed to go) in the head thats vital for tdci system.
the fuel pump requires an oil cooling supply on the tdci's your tddi may have the hole for high pressure oil and just require pipe putting in or it may be blanked alltogether.
heres some pics to show what i mean


for tdci system all wiringlooms in engine bay and behind dash would need replacing for tdci ones.
the turbos on tdci are vairable vain so again would need vac solonoid and wiring for it, or the wiring for the electronic box type turbo.
if however you had a 125, 135 tdci still you would have no problems fitting it all, and i think with some ecu tweaks the fuelling would be ok with slightly bigger capacity engine of the 2.4. About the 155ps ecu also if fitted to the transit things like cluster would not work. A mondeo pcm will start and run a transit tdci engine though and vice versa as ive tried it, just other problem is if mondeo pcm comes from an electronic boxed turbo car, it will not operate the vac operated transit turbo.
were the 125 and 135 tdcis you had 100% spot on, as i dont understand why youd get a tddi after them
i will be doing the 155ps 2.2 tdci engine conversion in my van hopefully soon, only thing i'm missing is the turbo.
oh also youd have to change flywheel to convert from tddi to tdci as pickups for crank sensor are different.