First thing to do would be to gain allies. The Redguards would be the first choice, they hate the Dominion. The Bosmer would be the second choice, since at least some of them are bound to hate the Thalmor. The Maormer may be worth looking into, but that runs the risk of causing massive collateral damage and possibly antagonizing the Psijics. The East Empire Company also able to be convinced to lend some of their ships, depending on how things are. The Sload would probably be a bad idea to try and enlist, as they're basically the TES equivalent of the Hutts, thus any deal made with them will probably end up coming back to bite you.
As for weapons, the Siege Crawler may be worth fixing up. I don't know the feasibility of rebuilding Numidium, since we've not really had much information on what's left of it ever since that whole Warp in the West business (especially considering the Underking presumably recovered his heart), but it would be an unstoppable trump card if you could somehow get it up and running. And of course, the Imperial Battlemages would be extremely useful, given the feats they pulled off during the admittedly disastrous invasion of Akavir, such as remote communication and enabling ships to safely sail through storms, in addition to their more conventional uses.