"C" is a velocity, not an acceleration.
Interestingly, one "g" (32.174 ft/s/s) equals 1.03 light years/year/year.
Merely accelerate at one g for one year and--neglecting relativistic effects--you will be "near" c and half a light-year out.
Each kilogram of mass (again neglecting Einstein) will have 4.89 x 10^17 joules of kinetic energy. Since one year is about pi x 10^7 seconds, this works out to about 1500 megawatts operating over one year to accelerate your kilogram.
To account for various inefficiencies, call it 2000 MW. Or two San Onofre nuclear power plants.
If you plan for any payload or structure on your 1-kg spaceship, and you plan to have the powerplant on board, the problem is reduced to stuffing two nuclear power stations in--say--100 grams and a few cc. Scale up until you hit 'Enterprise'.
The problem is that humans are too puny to deal with such power and power densities; it is like stuffing the Sun into the Rose Bowl.
Among the questions I mean to ask the Almighty is: "Why did You make us so short-lived?" and "Why did you put everything so freeping far apart?!?" It is almost as if the Universe is designed to prevent communication/contact among intelligent species--assuming that we are not alone.
Personally I have been forced to conclude that we are alone or very nearly so.
--Boris