Let's get the crud out of the way:
1. There is no stealth in space.
Thermodynamics say so. Check Project Rho for this has been debated and debunked to death. No stealth in space as long as you play by the rules of physics.
2. Decoys don't work. Any decoy that could fool your opponent will cost as much as a ship.
Newton says and thermodynamics say so. If your exhaust is of T-temperature and of L-luminescence, then you're outputting F-force. If F is smaller than what a ship would need then it's a decoy. If your decoy is as heavy as a ship and needs an engine just as big as a ship's you're better off building a ship anyway.
3. Weapons are more important than the platform they're mounted on. A space laser can have an effective range measured in light-minutes. Kinetic weapons and missiles have *unlimited* range. Provided you can push them hard enough, you could hit a target from half across the solar system.
4. Hitting is more important than destructive potential. If you can't jink you'll be hit.
There are four (and a half) kinds of weapon:
-Lasers (infra, visible, ultraviolet, X-ray). Their range limited by diffraction, but they'll have the best hit-to-miss ratio as they're light-speed weapons.
-Kinetics are rocks or metal thrown hard. Actually your biggest weapon is the ship's engine. Kinetic energy is a *****. You don't need a multi-ton railgun when your target's already on a reciprocal orbit of several hundred km/s. Kinetics have the worst hit-to-miss ratio as they can't correct their course and a maneuvering target can avoid them entirely.
They're the cheapest and *still* a weapon of mass destruction. Throw some big chunks of rock from higher up the well and gravity will give your rocks all the "OOMPH!" you need.
-Particle beams are the bastard child of kinetics and lasers, but in practice are more like short-legged lasers than kinetics. They have worse hit-to-miss ratio than lasers, but pack a bigger punch. (Ion canons are actually particle canons). They're c-fractional weapons that is most designs have their shot travel at a considerable fraction of the speed of light.
-Missiles fix the worst drawback of all your other weapons: they can react to your target as they get new data. They have practically unlimited range as you can build multi-staged or re-ignitable engines and can make them coast for a portion of their flight. Whether a missile can intercept a ship is the very same question whether a ship can intercept another. Both fall under the same restriction of space design (fuel-fraction, engine specific impulse, maximum thrust).
-Missiles could mount a variety of warheads and therefore use the best abilities of other weapons. You could mount nothing and let kinetic energy do its work however *hitting* a jinking target is really hard. You could mount a nuclear warhead, but with no medium to transmit the shock wave you'd still need to be within a couple of km-s of your target for radiation pressure to do the damage. Finally you could mount a bomb-pumped laser on the missile and get a weapon that both has an practically unlimited range and a light-speed weapon that's impossible to dodge if you're closer than a couple of light-seconds (the actual range depends on the target's ability to dodge).
-If you mount a conventional laser on your missile you end up with an automated ship, a "drone".
About weapon design:
-Kinetics could be nothing more than chunks of rock tied down and released from their harness.
-Particle beams would likely need a particle accelerator (either a helical or a linear design) so they'd be both bulky and heavy, since you can use magnetic fields to shape the particle stream it could have a small cross-section.
-Lasers will be big and have a great cross section as their range depends on the diameter of their main mirror used for focusing their attack beam. This could be a highly polished and expensive main mirror, a composite design that would be less effective but easier to maintain and repair... or even a throw-away external design that's only good for a couple of shots. Both have their pros and cons, but here's the main deal: if your ship uses a laser there's no logical reason not to build it as big as possible so it has a greater range. Therefore the laser will take up a great deal of your mission section. (While -as usual- propellant will take up the bulk of your ship).
Lasers vs Kinetics vs Missiles:
-There's a good parallel between the various weapons: A cop has a pistol that can hit a felon far away. This is the laser. He also has a shotgun that does a great deal more damage, but it can't hit objects far away. These are kinetics. (The picture is not entirely correct though as kinetics are highly speed dependent. On opposite orbits they're devastating... however if the combatants are on similar orbits they loose a great deal of their destructive potential). He also has a police dog, that he can set loose and it will track down the felon on its own.
The picture is correct as far as that the felon could also have all of these and he could shoot the dog with either his shotgun (kinetics) or his pistol (laser).
What's not accurate though is that lasers can also shoot down kinetics with impunity. The battle between "orange" vs "purple" has been long waged and the answer as to which is better: ...it depends.
A laserstar (a spaceship with a laser primary weapon) will likely be several magnitudes more expensive than a kinetistar (a spaceship with a kinetic primary weaponry). The quesiton whether kinetics have a chance depends on how much mass you can bring against the laserstar and how fast the later can jink.
If you're up to GW output levels of engines that can do full G-s of sustained thrust (torchships!), then kinetics have no chance... however in a more probably (for the early space future) mili-g powered ships with nuclear-electric engines they *do* play.
