Is there a limitation of speed of a spacecraft in space because of suns gravity? Is the speed purely dependent on the engineering? Or other factors comes into play?
Propellant: This is the main practical limitation on Δv, of the rocket's ability to change speed (go "faster" or "slower"). Assuming
conventional-matter engines, no exterior propulsion, and that you want to slow down again at a destination, the maximum speed you can approach is half the speed of light.
In-practice, spaceships tend to be unable to accelerate by more than a few tens of kilometers per second. Although we do have the technology to get up to a few percent of the speed of light fairly easily, these are mostly nuclear technologies which the main spacefaring nations are too timid to ever dare launching.
Relativity: You cannot approach or depart a destination at a relative speed of more than the speed of light.
Interplanetary/Interstellar Medium: If you move relative to the tenuous gas/plasma in space at a high speed, it can damage your spacecraft. Things like "hydrogen atoms" start looking more like "proton and electron radiation".