even with faulty plugs a JTD should be able to start normally with semi-cold weather. as long as the battery can hold enough grunt to get the starter up to a proper speed and the HPpump can deliver 300 bar it shoud start like normal.
if it starts to get difficult to start you can first try getting another car hooked up to the battery with running engine (and high idle for more power) and try doing a cold start then. if it starts good you know the battery is dying on you. if it is: get the most powerful one you can get fitted in the box. more power = better.
if it does not start better you need to check if the HPpump gets 300 bar startpressiure, if it does not or takes too long to get 300 bar it refuses to start. that might be the sign that injectors start to fail/leak or that the gasket inside the HPpump is broken. (cheap fix)
i usually recommend against replacing plugs as they thed to be VERY stuck and if they break while being removed by some farmer-mechanic it will require a new head on the block. VERY expensive.
Always cheat; always win. The only unfair fight is the one you lose.