The pros and cons of a proposed development near Nayland will be heard at an appeal inquiry on October 1.
A planning application by family firm Bunting and Sons for a Stour Valley Visitor Centre at Horkesley Park was rejected by Colchester Borough Council in February.
Buntings appealed and a government inspector will now hear expert witnesses and others representing the council and the appellant.
The company wants to develop 117 acres off the A134 at Great Horkesley to include a 101-acre country park, Suffolk Punch breeding centre, art gallery, art and craft studios, gardens, indoor display ring and other features.
Its recently-published business plan forecasts the project would attract 316,000 visitors a year, spending £7.5million on tickets, refreshments and souvenirs.
The cost of the scheme would be £12.9million, including £500,000 on a promotional campaign. The plan says the year-round attraction would create 146 full and part-time jobs.
But one of the main opponents, the Stour Valley Action Group, remains unconvinced.
Chairman Will Pavry said: “We have seen the business plan and are more convinced than ever that Horkesley Park would fail financially.
“Our opinion is backed up by external consultants employed by us to give a view of the viability and deliverability of the project. They conclude it would make a substantial loss.”
Other opponents include Nayland with Wissington Conservation Society, Dedham Vale Society, Dedham Vale and Stour Valley Project and the Colne Stour Countryside Association.
The 12-day hearing will be held at Langham Community Centre and Colchester Town Hall. The inspector will then report back to the Secretary of State and a final decision is expected in January.