It would be nice if we could choose a plane or helicopter suitable for the task in career mode to complete various missions.
There are currently very few planes and helicopters available. Not even the planes and helicopters that MSFS 2024 currently contains are available.
Maybe, maybe not. My guess is yes, but 3rd party (not on the marketplace) probably not and 1st party not any time soon (<6 months)
The sim is very moddable so who knows what will be coming (free or paid @marketplace)
Personal Comments and Observations - I am also a staffer on a Dev Partner
It’s up to the Third Parties to submit their aircraft for Career inclusion. Monday Night QB view - it’s early days for that given everything else that’s happened and what they’re doing with their own portfolios/roadmaps.
3rd parties on the marketplace, I made a mistake by thinking only the ones outside (FNX,F1 etc) because I thought that was the meaning behind the question. So i’ll edit my reaction. 
Given the many, many problems with Career mode, I would hope that they extend career inclusion to third party, non-Marketplace addons, not just Marketplace. I suspect they will lock it to the Marketplace to keep things “fair” but let’s face it… nothing about Career mode has been fair to players. I do suspect it would be up to devs to make it compatible, but I’m not buying an addon I already own for full price on the Marketplace just for Career Mode. I’ve mostly shopped outside of the MP, and that is not because I wanted to, it’s because the MP has historically been very sluggish and has limitations on weapons. I’d hope I’m not retroactively punished in FS2024 for having bought most planes from other vendors.
What about add-ons to aircraft already available in career mode? Like variants adding an ag-nav computer like this:
https://flightsim.to/file/89968/agriculture-sat-nav-and-light-bar
When can we use them in career mode? Career mode is fundamentally broken but add-ons like this can make it enjoyable, why we are not allowed to use them?
No idea on that. Like I said, it’s early days, I have yet to see any Third party get certified on Career, never mind add-ons.
What about the rest of the default fleet that isn’t in career mode are we getting those?
Like the 747-8, imagine having a career mode without the jumbo jet
Career mode is half baked, they really need to get more plane variety and all the liveries included. Currently grinding between like 5 aircraft and 1 paint scheme is becoming boring very very fast
Carenado has released the C-210 Turbo. Unfortunatly in can be used only for flightseeing. It would be cool for transport use.
Greetings Uli