I would think that a Plug N Plate nickel kit would suffice.
See
http://www.caswellplating.com/plugnplate.htm
>>Is etching all that is needed to be done to the old plating in order to replate it?
Yep - just acid etch the exisiting nickel so that the new nickel bonds properly onto it.
>>There are several keys where the underlying brass is showing through. Will this affect the evenness of \'color\' of the new plate?
No - it will plate over the brass easily and you should have a uniform color.