You can unlock everything through the skill and research trees. Its p2progressfaster which is essentially paying 2 win. I don't know if there is a cap for things like backpack/vault space. So essentially if you buy into the founder's you may have a higher cap than people who don't.
Even after putting a good amount of time into the Alpha I couldn't tell you if there is truly an end game. A lot of the game is several types of mission where you explore, gather materials, build, defend, and repeat. There is just different goals, map layouts, and mob types.
Regardless of that, there is so much content that getting to end game would likely take a long time, though that content is repetitive. You just get new heroes, skills, guns/traps, and mobs, but the of explore, gather, build, defend is always there.