Shorter boats don’t track as well, and don’t go as fast. Considering you are going to cover a lot of miles, having a boat that helps you move straight forward more efficiently would really pay off. Matt Krizan’s paddle of California coast was 36 paddling days, and many of those days were in excess of 40 miles. If you get a boat that can average an extra mph (say 4mph instead of 3mph), you can see that this can greatly shorten your paddling time.
On super light packing, keep in mind that there will be stretches where you need to go a week plus without being able to restock. This often includes water (though this year there are lots of streams running which you could possibly filter from, but that isn’t the norm). So your gear (tent, sleeping bag, clothing, etc.) isn’t usually the majority of what you carry, but instead it is food and water. And you will need lots of food to offset the calories you burn.
Another consideration for a $800 kayak is that you would be looking at a rotomolded plastic boat. On plus side, they are cheap and durable. Downside is that if you did wreck one, they are generally not repairable in the field (and limited repairable at home). Most everyone on these extended expeditions use composite boats, because you can repair them overnight in the field (though they are a bit more fragile, so additional care on launches and landing needs to be taken to avoid needing to make repairs).