Carbon taxes? MOT taxes? I think there's some confusion here. The only tax that is relavant to engine size and emmissions here in the UK is the Road Tax (VED). Older cars like E34's will be exempt to the CO rates anyway, and you will only have to pay the one tax rate. MOT cost is the same no matter what car you have. The only other factor to budget for is insurance, as normally the higher the engine CC the greater the annual premium.
HTH,
Shaun M