Go for your challenge, have fun . Did you look at https://cmdrs-toolbox.com/fleet-carrier-calculator
It looks to me you can have a year of fun with minimal service cost hassles if you buy in when at >7.5Bcrds
So, my thinking is 8Bcr is the trigger point. As it leaves some reserves for jumping around moving your stuff into the carrier.
You get a full functionality carrier that you can test various services with.
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Interesting. I have just over 9bn so would I be right in thinking this is viable amount for modestly fitted-out carrier? I haven't really looked into it much to be honest - I'm not even sure what modules I'd need. Restock, repair, refuel... exploration, the ability to mine, obviously.
I need to research more!