The fleet is currently maintained and tested by private contractor AssetCo and leased by the county council.
The authority has moved to end the 20-year contract as it was "not delivering what we expected" - but emphasised emergency cover was unaffected.
Negotiations over the sale have begun and officials said the money would be found from existing reserves.
AssetCo Lincoln and fire bosses signed the contract in 2006 and then announced the acquisition of 35 new engines and 22 refurbished ones.
The council said it cannot go into details why it ended the contract for legal reasons, however parent company AssetCo Plc has struggled with financial problems.
The council's assistant director of resources, David Forbes, said: "The £9m has been set on the basis we think that is an absolute maximum cost, we think it might be less than that.
"Until discussions with the bank are resolved, which is tied up with separate legal issues with AssetCo, we won't know that."
The authority added it believed the cost of maintaining the fleet in-house would be the "cost neutral" compared to the leasing contract.
Earlier this week the county council said it had underspent by £23m, mainly due to it making about 1,000 redundancies.
AssetCo, which also provides fire equipment for London, said it could not comment ahead of commercial negotiations but has previously stated it was moving away from UK maintenance contracts.
Up to £9m is to be spent buying the vehicles used by Lincolnshire's fire and rescue service.