Kakuzu is one of the stronger members due to his experience and the sheer variety of high level ninjutsu he can use. Not because of his fire power. Kisame is actually stronger than Kakuzu and can quite literally murder him in a straight up battle. Coming to the point,
1. Tough one to say really but base Jiraiya has crazy toad techniques like summoning the mountain toad's stomach or trapping his enemies in a house sized toad etc. Itachi had to resort to Amaterasu to even break out of the toad mouth trap. If Kakuzu gets trapped, he might be killed. But still because of Kakuzu's sheer arsenal of dangerous techniques and his experience, it'll be a 50-50.
2. The same scenario, but Jiraiya is allowed a power-up which only tilts the odds in his favour. 70-30 in favour of Jiraiya.
3. Jiraiya wins. Sage mode is way beyond anything Kakuzu can handle and the crazy summons, Ma & Pa and the strength, speed and reaction amp from senjutsu is just too much. One ultra big ball rasengan can honestly seriously damage multiple masks at the same time. Allowing Jiraiya to start in Sage mode is not fair in anyway. 100% in favour of Jiraiya in this scenario.