If a director is dismissed/resigns - and at that point the company has a certain amount of debt, if the remaining directors buy the shares - are they buying the leaving director's portion of any current debt liability?
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A director (of a Ltd Company) doesn't have any liability, unless they signed an agreement to personally guarantee a debt. It is the company that is liable, which is the whole point of a Ltd company. Additionally a Director might be liable if they had done something illegal that resulted in the debt but otherwise the debt stays with the company when you resign.
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What if the company were to go into liquidation a few months after the director left?
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The termination agreement should make it very clear what is being bought within the shares. This should be a legal document with indemnities sought where possible on all outstanding debt issues as this not automatic. Recent case law has shown personal liability of a director in signing contracts.
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