From the information you have given us, we consider that the ECI clause is not suitable for this work, whatever main option is chosen. The clause enables you as an employer to appoint a contractor (not three contractors) to design most of the works or to advise on the design carried out by others before work starts on site. It then enables you and the contractor to agree a price for carrying out the main construction work and, if you fail to agree, contractual obligations cease at that point.
In any event, ECC Option B is unsuitable for design and build works unless the design is for things like temporary works or minor items. That is because the eventual design will dictate the quantities of work carried out and that means the bill of quantities cannot be accurate when the contract is entered into. Furthermore under ECC Option B, changes to quantities are entirely your risk, so the contractor could potentially be incentivised to over-design the works because the more quantities it produces, the more it will be paid.
Option E is usually used for the first phase of the contractor’s work in this sort of scenario, that is the design, because at that early stage it is very difficult to predict the amount of resources needed accurately. Once a price has been agreed for the construction phase, ECC Option C is chosen because it is a risk-sharing contract that best suits the ethos of ECI – that is, you and the contractor working together to try to achieve your objectives, usually including your budget.