I believe you also have 2 other options, depending whether the agenda for the AGM has been issued or not.
Schedule 1, Clause 4 (1) states any owner may require a motion to be put at a general meeting and Clause 4(3) states the Secretary must give effect to the requirement. You could require any part of capital works to be quoted by, say 2 companies, and in writing.
Alternatively, Schedule 1, Clause 18 provides you may amend a motion at the meeting provided it is a motion for which notice has been given. Suitable wording to amend the ‘blank cheque’ to quoted would be required.
Both these options would probably require some lobbying before the meeting to outvote the SC.