I know it's a bigger step than you intended but have you considered a new base unit?
It might be time, spec wise, to call it a day on the unit in question. By the time you add up the individual costs of the parts a new unit my be a better bet (no compatability issues etc).
There are many companies that can supply 'base unit only' systems with a spec far higher than you are thinking of for about £200. Even less if you consider 'second user'.