Might help if the OP posts pictures and a description of the issues.
Personally, I replaced my CUs with all RCBO ones a few year ago so that the installations got a completely "clean" EICR. While not required by regs, there are two reasons I did it :
- Safety - there were no RCDs before
- CYA - if anything should happen and a tenant tries to claim it was my fault, I have a magic piece of paper which shows that it was in good condition and fully compliant with regs (as they were at the time). That raises the bar somewhat on their burden of proof in trying to make something my fault.
So there is an element of your attitude to risk - both in terms of electrical safety, and in having a tenant claim that some issue was your fault. Having all circuits on RCDs is pretty well a no-brainer (as our US friends say) these days, all RCBO is definitely not a bad idea.
Note that it's possible to have a fault which will trip a shared RCD even if the MCB supplying the circuit with a fault is switched off - RCBOs eliminate that failure mode as well.
£600 sounds a bit steep to me - but it depends on the work required and how many circuits there are.
EDIT: Just to add, the most common installation is a board with a main switch, two RCDs, and MCBs in two groups. When an RCD trips, it will take out (around) half of the circuits and part of the planning is to work out the least dangerous combination - some like to put upstairs sockets and downstairs lights on one RCD, and vice-versa for the other, so that if the lights are out you can use table lamps etc.
Going all RCBO means that tripping the socket circuit (probably the second most common) won't take the lights out. Just don't have outside lights on the same circuit as the inside lights - I suspect damp in outside lights is the most common cause of nuisance tripping.