The simple fact of getting an object up to relativistc velocities requires reaction mass (boy, does it sound like I might even know what I'm talking about?..;) of vast proportions. In fact, any reaction-engined ship is going to be mostly fuel tanks.
To get to the stars, you'll need to both go as fast as you can, and go for decades or centuries to get there. It becomes a chore to create a ship that big, so, as in ages past, a hollow asteriod is constructed, and filled with fuel. Quarters and machinery to sustain generations of colonists. Here's the Centauri Bound, a nickle-iron asteroid pulverized on the inside. The engines are a hybrid..initially plain 'ol rockets to get the asteroid moving in the right direction, then cyclotronic-ejection systems propel the powdered iron to about half-lightspeed, which over nearly a hundred years, would propel the 'big tater' across it's path to Alpha Centauri.
I know I need to get away from the Discombobulate 'plugin' in Blender, but it's just so useful. This is a previous model run through Blender, then textured through Wings 3d. Textures and lighting in Bryce..enjoy..;)