If we assume a current level of technology, game rules generally don't give realistic results. We can use real rocket science instead. Ground to space is probably best handled with an aircraft launch vehicle that lifts a rocket to the edge of space, where the rocket takes over (and the aircraft returns to the ground). Return requires heat shields, like the ablation shields of early space flight and the reusable tiles of the Space Shuttle. Almost all of every surface launch mass is fuel.
Interplanetary travel has lots of options: chemical rockets, fission thermal rockets, ion drives and similar electric technologies, and light sails.
There is no stealth in combat. Missiles are the only attack. Dodging is the only defense. Throw enough stuff at an enemy and they're dead -- either from catastrophic impact or from fuel exhaustion that leaves them on a long flight to nowhere until their ship equipment runs out of life support.
The rules of low technology space combat are pretty simple, but they're not much fun. Best answer is to fight it out on the ground.
