Which wet suit should I bring with me?

This depends on the location.

Pemuteran, Tulamben and Gili Trwangan: from 3 to 5 mm (water temperature is usually between 27 and 29 degrees)
Nusa Lembongan: 5 mm if you dive only a couple of times, but if you are planning to stay longer and you usually suffer cold, you might also enjoy a 7 mm or at least bring with you a hood (water here can also drop to 20 degrees!)