Generally speaking, I recommend buying good quality parts for your bike, as this might save you money in the end: They last much longer and your decrease the risk of waiting several days in some hotel for the expensive shipment of the needed part from overseas.
You need to bring:
—Tire levers, patch kit and glue: Yes, some lucky bastards never get punctures but you likely will.
—Extra tube: Some punctures could be very bad and a spare tube will keep you rolling. After 5-10 patches, you might want to change the tube anyways in case some of your patches leak a bit
—Oil: To keep the chain lubed
—Multitool: With the needed allen-keys, a screwdriver etc. Some fancy ones come with a chain-tool and a spoke-tool, nice combination.
— Plyers and adjustable spanner: For tightening bolts and nuts.
— Couple of bolts and nuts: 4mm and 5mm allen-head bolts and nuts in various lengths. Rarely take up any space and weight but could keep you happy when a bolt break on your racks.
—Duct-tape and plastic straps: For interim-repairs, not only for your bike but also for your panniers, tent etc.
You could consider bringing:
Some of these items might not break, but if you plan on cycling in regions where you have little access to good quality spareparts, you might want to bring them along as they need replacement at some point and the local cities doesn’t store them.
— Tire: Some punctures can be very bad and cut your tire completely, though it is rare.
— Spare chain + chain-tool. If you have a shimano-chain you can fairly easily remove the broken link and assembly the chain again, if it should break (and if you carry a chain-tool, which normally is small and light). I’ve broken my chain once while touring and consider it to be a very rare event. A spare chain can be heavy.
— 3-5 extra spokes + a spoke-tool: If you plan on cycle on lots of poor quality gravelroads, this could be a good idea. Some people break dozens of spokes, some never break a spoke: At the time of writing, I’ve cycled 55.000+ km and broken one spoke only. I think broken spokes are caused by a) poor quality spokes and wheels b) reckless cycling and high speed on poor bumpy gravelroads c) luck (or “un-luck”, if you will). If you carry extra spokes, do not store them in your panniers where they’ll get bended, instead tape them to your frame.
—Chain-whip and lock-ring nut: To take the cassette apart. If you break a spoke on the cassette-side of your rear-wheel, you need these tools. Chain-whip can be heavy, I cycle with one and I hate it, but I’ve had to use it a couple of times and then I am happy that I carry it.
—Brake pads: They don’t really break, so it is a matter of your brake-pads life-time, your planned distance and access to new ones. However, they tend to be light and small.
— Gear cable and/or break-cable: Light and small, though your cables very rarely breaks.
— Needle and thread: Fix your tent, clothes etc. But you can easily buy a needle and thread in the next city if it happens. Anyways, super light and small.