All new Plaster needed for unmodernised Victorian Terrace?
Impossible to answer in general, and hard to answer in specific without actually seeing it. Photos would help, but seriously, I'd suggest you bring in a pro (or several pros) for advice and estimates. After they tell you what the options are for that specific situation, you can decide how much or how little you want to do yourself.

In the US, I'd suggest having this done as part of the inspection process since that estimate can become a bargaining point when setting the final price of the house.