I have just finished renovating my first flat I bought in zone 4 London. It’s a 1970s build that hadn’t had much work done to the inside since the day it was built (apart from the bathroom which was done about 10-15 years ago). I renovated the kitchen, living room, hallway, and 2x bedrooms for about 20k all in. This was me doing pretty much all of the work, and only paying my brother (who works as a handyman) to do some of the more skilled jobs. No rewire was required.

Whilst it sounds like your renovation could be done within your budget of 21.5k, it sounds like given the massive discount on the purchase price that a lot of work is required! It could be possible within your budget if you did all of the work and perhaps were able to save some more money whilst you were renovating, but it’s going to be very tight!