whats your brief for the job? if the customer wants em flat youre prolly lookin at hacking off and rerendering, nothing to stop you d+d over if its sound enough..depends on the customer...
keep one eye on the pro's/con's of a method though and point it out to em if you need to..like church says, old buildings are usually built with lime mortar and to do the job buy the book you need to be looking at sympathetic renovation...pros - will breathe/move/keep with the program... cons...bloody expensive - limelight is well over a tenner for a 25kg bag, sand is 25 - 30 quid a ton..
church is the man for rendering but i aint to sure exactly what the make up of these renovating plasters are but its something like sand/cement/lime...in what proportions i dunno, and nor with what additives...
i reckon you should be able to meet the two in the middle if you need to keep the cost down..i.e. make your own renovating plaster...what you reckon church??