A restoration is a set of works you buy on the assumption the roof underneath can hold them. That assumption is what an inspection is for, and it is the part most quotes skip.
Start with what actually holds a tiled roof down. Under the National Construction Code, roof tiles must be secured by galvanised clout nails, self-embedding head screws, purpose-made clips, or flexible pointing complying with AS 2050 — one of those, or a combination of them. Read that list again: pointing is a fixing, not a finish. The code sets how a roof is installed, not when it stops being restorable, so it gives you no threshold — but it gives you the question. How is this roof fixed, and is that fixing still doing its job? A coating cannot answer a fixing problem.
The next question is where the failure sits. Bedding and pointing are two different layers: the mortar bed carries the ridge and hip tiles, and the pointing seals over it. Ask any inspector to show you which one has gone, and to put the answer in writing, because a quote that reads "re-point" when the bed underneath has moved is a quote you may end up paying twice.
Then the tiles themselves. If your tiles are terracotta and older, ask about fretting — the erosion of the tile body, which the Australian Roofing Tile Association addresses in its own technical material and answers with replacement rather than with a coating. Tile condition has to be established before a cleaning method is chosen, rather than the other way round.
Almost every Boondall reader owns a whole house roof — 83.0% of occupied private dwellings here are separate houses, and roughly seven in ten are held by an owner-occupier — so the decision tends to be made once, alone, and from savings, at a median weekly household income of $1,887. That is the reason to spend on the assessment before the works, not the reason to hurry.