Slow cookers - capacity and mess

Is everything sealed or is it like when cooking normally?

Slow cookers are definitely not sealed. It'll release odor while cooking (which, depending on what you're making, might be pleasant!) and there's no way to get around the potential mess of transferring food to/from the cooker. You'll want at minimum to have a relatively clean, clear area where you can fill and use the cooker.

That said, the enclosed and relatively weak heating elements of slow cookers generate less excess heat and can be used outside of a kitchen without too much risk. Make sure the surrounding area is relatively clean and free of flammable material for safety; make sure your electrical outlets are likewise in good working order.

do I need to add water or can I use 6.8 Liters purely for food?

Check out this related answer - you need some sufficient level of liquid for just about any slow-cooked recipe. I would advise you to carefully follow a recipe at first rather than going off-script and trying to develop your own. You'll be better assured of success this way.

(That said: yes, 2 kilograms of chicken should fit in a cooker this large.)

I want to save time

Keep in mind that they don't call it a "slow" cooker for nothing. These devices are only capable of low, slow heating, and can only be used for recipes that take extended low-temperature cooking, usually at least a few hours. They can't sear, or saute, or boil; they are by design incapable of any fast cooking method. Those methods can be key to developing flavors (browning meat is often quite important) and although you can usually cook safely without them, you don't want to just skip those steps if they're called for in a recipe.

Slow cookers can be convenient in that they can safely run for several hours without being actively tended (unlike, say, a pot on the stove) but how and where this will actually save you time depends on how well you can plan your cooking ahead. If you have an unpredictable schedule, limited refrigerator space to store prepared food, or simply don't want to eat the same thing a few days in a row, a slow cooker probably won't save you time, money, or aggravation.

As noted by @Catija, you could consider a "multi-cooker" as an alternative; that would broaden the variety of cooking you can do, allowing the options of searing or sauteeing whether called for in a slow-cooked recipe or just if you're in a hurry to make dinner. Many of these devices will allow for pressure cooking that cuts down on overall cooking time, and they have enclosed heating elements and other safety mechanisms. Keep in mind though that this would not minimize the mess, odor, etc. of those cooking methods. Oil can splatter whether you're doing it in a pan or a fancy multi-cooker in your bedroom; the same applies to the odor, smoke, and noise involved. These devices are also generally more expensive than a traditional slow cooker, so if you have budgetary constraints you might want to try a cheap slow cooker first, figure out if it works well for you, then upgrade later on.

You can fill it to within about 2cm of the top (1"). That should be plenty for cooking 2kg of chicken that will pack in, even if you're adding liquid.

You don't need to add liquid, though many recipes are quite wet. For example you can cook a whole chicken.

At that volume you might need to freeze. It might be easier to cook a smaller amount like every 3 days. It would also create less odor.

Unless you have trimmed the fat the fat will be plenty of liquid.