Regardless of costs coming up, as buyer, I would be put off by crap management company who cannot clearly specify costs coming up soon - shows that they could be very small or have no limit and be huge. Even if they eventually give the info now, I'd be worried about stuff they might later dream up.
Doesn't help you Op, but a key reason why people don't like lease house houses (no longer allowed to be built) or houses with estate charges. Standard freehold house on normal road which is maintained by council and doesn't need strips of grass or ponds maintained is better.
I would ask your conveyancer if the management companies refusal to give the pertinent information within a certain timeframe becomes a breach of some kind of law. I think when people are buying leasehold, Freeholder's do have a time frame within which they have to provide the information.
Essentially, they can say there is no work planned and then not give a figure obviously, but if they say work is planned, they need to give a figure or a range if they are not totally sure yet. They ought to be able to list the planned work and that in itself should suggest if it's large and expensive or small scale.
What does the agreement you have with the management company say about extra works? Is it totally open-ended about how much these can be? I'd so, that is terrible and as a buyer I wouldn't go near it wih a bargepole.
Re indemnity insurances, I'm not sure you will get one for this. Such insurances usually cover things like clauses in leases which aren't specific enough or works which have already been done and not certified. They simply pay out if the owner is sued for a breach and not like standard insurances to cover costs which might come up. They nearly always have a clause which requires the holder of the indemnity to not mention they have it - because otherwise interested parties might start making claims when really there is no issue.
I agree that ringing up and talking to conveyancer and asking them to tell you what their plan is to get a resolution is key - don't be fobbed off. They must have seen this stuff before and it is down to them to push the solicitor of the management company to get the answers you are entitled to. Unfortunately though this might take time and mean you lose your buyer ...but if you at least have the info before getting another buyer, you shouldn't have the delays next time round.