Cars flickering lights (internal and external)?

I've been having problems with my ford fiesta 1.4 04 plate its recently been making a repetitive squeaking noise and both headlights and internal lights (radio and overhead light) keep flickering there seems to be no pattern but always does it. I've had 1 instance when the car turned over but wouldnt keep the the revs so i'm in 3 minds whether its the belt the alternator or the battery. Any helpful responses would be great and would a alternator and belt be easy to replace myself?

Belt is straight loose on that alternator hehe
Slipping, making the noise
Slipping, not turning/charging

Replace cheapest to more expensive… Belt, then alternator, then battery

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