McThag had a post a bit ago on the realities of space combat. Just like the old days of sail, just further away.
Things get even more complicated when you are dealing with a fixed amount of Delta V, as with chemical rockets. Because of the vast distances, lack of cover, enormous speeds, and limited mobility of lower tech spaceships, combat is less like the thrilling dogfights you see in movies and shows, and is closer to a game of chicken crossed with dodgeball with a side of Marco Polo where nobody's speaking up.
At earlier tech levels, missiles are the go-to, high weight per shot for low carried weight of something like a massive gun, with the option of guidance to deal with the sheer distances involved. CIWS type defense systems of any variety will be generally more relied on for defense than mobility and maneuverability in all cases involving ships that aren't specifically built for such things (fightercraft, and even those will be a lot less maneuverable than you'd think. Closer to Torpedo Boats than fighter aircraft. See more here. The key advantage of smaller, purpose built battlecraft is being tailored for a low moment of inertia, to allow the ship to be spun in order to perform maneuvers. Only really viable at tech levels lacking in laser weaponry or similar. The other key benefit of carried battlecraft is having ships who's delta V is all dedicated to combat maneuvering, unrelated to that required to complete the actual travel required for the mission).
Ship to ship actions, the true stomping ground of the marine, will remain necessary. Lacking tractor beams or similar, and given the limited fuel for maneuvering when working with DeltaV limited systems, this is another area where harpoons and grapnels come into play. Boarding ships will have dedicated braces to absorb impacts with the target craft, and heavy armor to withstand abuse taken during the chase leading up to the boarding action.
Dropships are a special case, serving as a small craft to ferry marines and supplies to and from the surface. Given that most celestial bodies of a size that will present substantial gravity to be escaped have atmosphere, dropships will be fitted with folding, adjustable wings. This will allow them to take advantage of the lowered fuel requirements for climbing through the atmosphere space-plane style, before the final boost out clear into space proper. Fuel processors, if available and required, will be fitted to these ships. Finally, for operations smaller than major scale landings, the dropships are likely to be the only non-ortillery (orbital artillery) support available to the ground forces; as such, rocket/missile artillery is likely to be installed, with a small CIWS system, and moderate armor to survive both the landing and serving as a ground base. To make up for the lack of support, a dropship drone boat full of automated mortar systems and recon/attack flyers may be sent along.
Overall, ships intended for combat troops will tend to have even fewer creature comforts than most realistic spaceships, by virtue of nearly everyone on board having space combat armor, as opposed to the simple skinsuit space-undies used by most spacers. Systems for switching out used air with fresh while still suited are dotted around the mostly unpressurized ship. Suits are kept mostly topped up, in case of emergency. The main ship's arsenal will be intended to both serve against other ships, and provide ortillery for those on the ground.
This got long and rambly and I'll probably re-do it later with numbers and things.
Many thanks to various other people who have been referenced in the writing of this post, such as Erin Palette's Traveler Tuesday, Atomic Rockets, McThag (for his various also-traveler-tuesday posts, including the one linked at the start that included mention of Atomic Rockets) in particular.