I wouldn't limit yourself to kit and kilo kits like coopers, you may be dissapointed, they may not store well, they may get boring for you etc.
If you are going to do this I think you should be able to do all types of brewing, based on the available ingredients...That means being able to make great beer with kit and kilos, extract with grain kits, but also be able to brew Ag as well....That way you can be open to whatever ingredients you may stumble upon.
If you are sticking with 2.5 gallon batches, you can get up to 14 pounds of grain in a 5 gallon cooler with a basic SS braid, obviously more with a 10 gallon cooler.
And you can boil down to 2.5 gallons in a 5 gallon pot, especially if you use foam control.
That way if you come across malted grains on your travels, you will know what to do with them.
Also I know you are asking about carboys and buckets...I think for your situation, where space will be an issue, the best options may be the 3 gallon better bottle.
The nice thing is their square size and you can fit several in a small space...I don't know if you can tell by this pic, but I could easily put one in front of the other and ferment about 6 in the rubbermade bin I use as my swamp cooler.
Another option, both as small fermenters and airtight storage for grain, and extract are Gamma Plastics Vittles Vaults available through Petsmart. They are food grade, stackable, come in various sizes, and have an airtight locking lid...
Many people drill a hole, add a gromett and slap an airlock on for a fermenter.