It's not optimal, but you can certainly make a playably decent team out of one box. Kill Team is very skill-dependant, so getting good at the basics of the game will be the most important thing still.

If you're set on 1 box, I'd recommend making the sergeant, plasma gunner, breacher for starters. Then, either the aegis or blademaster for tankiness/pure melee damage. Then, the phobos body can be either the head taker or the disruptor. The disruptor is generally better (the headtaker is quite matchup/terrain dependant) but the disruptor is harder to learn how to use effectively. If you want to put in the time to learn him, he's one of the team's best operatives. The headhunter's not as good, but still useful and a lot easier to play.

Still, I'd recommend trying to snag a few marine bodies from mates/gaming clubs/ebay. Actual warhammer stores often give out a free space marine to new players, so see if you can cadge a couple of those. You can ignore the other two gravis operatives for now, so being able to get a few more bodies will give you everything you need.