That could lead to much more research.
For ICE vehicles a few areas come that to mind:
Regulatory Standards. A study of the recent history of EPA, CARB etc emission and GHG standards and what class vehicle they apply to could be useful.
For eg I think 2017+ new emission regs were applied. That might make a difference to you when choosing a 2016 vs 2017 model year. Or, for eg, is flex fuel or E85 important ? If so when did that come in.
Compliance: Where and how can you have a prospective vehicle for purchase tested for emissions output. Are there DIY handheld devices you could acquire to check periodically ?
Vehicle Equipment and Features. Do some prospective vehicles have feature that would contribute to your goal ? For eg. stop-start, live fuel consumption information, enough interior cargo room to not need added exterior storage, low resistance tires, etc.
For trailers, frontal area and drag. How does towing a prospective trailer change emissions for a prospective TV.
User Operation and Practices. Could research best practices re your criteria for such things as idling, engine warm up duration, acceleration rate, ambient temperature best operational limits etc.
They're may be devices that could help monitor results, such a combustion byproduct readers, temperature gauges etc that could be installed.
Speed has been mentioned, how about terrain, time of day. Do certain vehicles produce more emissions climbing hills than others etc.
On that note future route planning might help re reducing emissions - like hyper milling
As for EVs, that's a whole 'nuther thing. Just considering operating, one could research recharging power generation source and there own emissions contributions.
Again, future route planning could help the cause.
That's just some ideas, and just the tip of it I'm sure.
Lot's to research and consider ahead of you. Good luck with your decision !