MEL
In the case you cite that would be a matter to discuss with dispatch, and the reasons you cite for concern appear valid. If your grounds for declining the aircraft are rooted in concern for the safe conduct of the flight then your position should be sustainable, that is what you are paid for, risk analysis and management.
These matters usually come to a head (in my experience) when there are multiple MEL items which are manageable on a nice day, but could compromise safety in less than optimum conditions were another failure occur to ramp up the workload.
Fuel ? per previous, if there is a sound operational reason for carrying extra, carry it.