I willwork –future simple: neutral future or future facts / instant decision (at the moment of speaking) / prediction based on what we know, think, assume
I am going tobuy this – to be going to: a future intention, i.e you have decided to do something in the future / a future prediction based on evidence
I will beworking – future continuous: something will have started before some point in the future and will probably carry on after it / future without intention, i.e. something that will happen in the ordinary course of events.
I will haveworked – future perfect: something will be completed before a specific time in the future
I will have beenworking – future perfect continuous: an action continuing for a period of time up until a point in the future, the emphasis is on the length of time