This is all sorts of awkward--I think I'd want to do about two more passes, and get further from the literal, before calling it smooth and readable--but it's roughly grammatical in English now. I think.
1.3.1 Led by these things and stirred up by Orgetorix's announcement, they decided to prepare the things which were suitable for marching, to buy up as many pack animals and wagons as possible, to do as much sowing as possible, so that on the journey plenty of grain would be available, and to strengthen peace and friendship with the nearest states.
1.3.2 They were convinced it was sufficient for them to do these things for two years; they established with a law the departure (from them) for the third year.
1.3.3 Orgetorix was chosen for doing these things. He took upon himself the legation to the (other) states. He persuaded Casticus Sequanus, son of Catamantaleodis, whose father had for many years been ruler in Sequanus and was named a friend of the Roman people by the senate, to seize rulership of his own state, because his father had possessed it before;
1.3.4 likewise he persuaded Haeduus Dumnorix, brother of Diviciacus, who at that time was a prince in his state and had been supported by most plebes, to attempt the same thing (as Diviciacus), and gave his own daughter in marriage to Haeduus.
1.3.5 He thought it good that creating the conspiracy was very easy by this deed, because he himself would maintain rule of his own state:
1.3.6 (he said) there was no doubt but that the Helvetii would have power over most of all Gaul; he confirmed that his supplies and his army would be brought together for that rulership.
1.3.7 Because of this speech the men (who'd been) brought together gave their word to each other by oath, and were hoping that with the three most powerful and stable tribes of all Gaul, they would be able to get the seized rule.
1.3.1 Led by these things and stirred up by Orgetorix's announcement, they decided to prepare the things which were suitable for marching, to buy up as many pack animals and wagons as possible, to do as much sowing as possible, so that on the journey plenty of grain would be available, and to strengthen peace and friendship with the nearest states.
1.3.2 They were convinced it was sufficient for them to do these things for two years; they established with a law the departure (from them) for the third year.
1.3.3 Orgetorix was chosen for doing these things. He took upon himself the legation to the (other) states. He persuaded Casticus Sequanus, son of Catamantaleodis, whose father had for many years been ruler in Sequanus and was named a friend of the Roman people by the senate, to seize rulership of his own state, because his father had possessed it before;
1.3.4 likewise he persuaded Haeduus Dumnorix, brother of Diviciacus, who at that time was a prince in his state and had been supported by most plebes, to attempt the same thing (as Diviciacus), and gave his own daughter in marriage to Haeduus.
1.3.5 He thought it good that creating the conspiracy was very easy by this deed, because he himself would maintain rule of his own state:
1.3.6 (he said) there was no doubt but that the Helvetii would have power over most of all Gaul; he confirmed that his supplies and his army would be brought together for that rulership.
1.3.7 Because of this speech the men (who'd been) brought together gave their word to each other by oath, and were hoping that with the three most powerful and stable tribes of all Gaul, they would be able to get the seized rule.