The Carrera is a cartridge pen and takes the short standard cartridges sold everywhere, such as those offered by Pelikan among others. Montblanc's own cartridges fit as well, but cost considerably more without anything changing in the way the pen writes. The choice is thus purely a question of price and preference; more important than the brand is that the ink is fresh and the pen is used regularly.
If the pen is being brought back into use after a long break, flush it through with lukewarm water beforehand until no more ink comes out, then let the nib and feed dry before the new cartridge is fitted. Dried ink in the feed is the commonest reason why a pen that has been stored for a long time refuses to start. The Carrera belonged to the plain range of the 1970s that Montblanc ran alongside the Meisterstück at the time; it has no collector's value worth mentioning, but as a sturdy everyday fountain pen it is entirely serviceable.