What is the expected temperature at your race? If the water temperature is above 76.1 degrees Fahrenheit (24.5 degrees Celsius) wetsuits are prohibited anyway which takes that decision away. If you can wear a wetsuit, are able to afford one (a cheap one will do) and can train with one I would recommend it unless you are an excellent swimmer and already know you’re better without one. I find it’s a bit more taxing on the shoulders to swim in one but the additional buoyancy is a god send if you have sinky legs like me
Where I am we have a few wetsuit mandatory races and others where the water temperature is low enough to often make them mandatory. I don’t think that’s usually the case with IM branded events everywhere though…
As to whether or not you should wear a trisuit; you’re doing a very long event which means that your transition times are less important than in a short race. You can put on full bib shorts for the bike and running shorts for the run (so long as you are not publicly naked while changing = disqualification) if you want to prioritise comfort. Do you get a changing tent, or are you allowed a towel/dryrobe in transition?
To echo the others about the running shoes; nothing is better than spending time in shoe shops and figuring out what works for you through discussion and testing.