Check the plate before you buy anything: a Miele name on the box can cover more than one version. The data (rating) plate with the model and type code is on the underside of the vacuum cleaner. Quote the model type from it when ordering bags or filters.
Don't judge a bag by how full it looks. Fine dust clogs the paper or fleece long before it's packed, and suction drops off with it. Trust the indicator over your eyes.
If the indicator says full with a nearly empty bag, look for a blockage in the hose or floorhead before you swap it.
Only rinse a filter the maker says is washable, and let it dry right through before it goes back in. Most of the "it smells" complaints start with a damp filter.
Quick answers
Which dust bags fit the Miele Compact C1?
FJM, which Miele sells in 3 different packs.
If I've got a Miele Compact C1, which HEPA filter do I need?
SF-HA 50, or any of the other 2 Miele versions listed above.
What exhaust filters does the Miele Compact C1 take?
SF-AA 50, or any of the other 3 Miele versions listed above.