If you want your contract to provide a project which is ‘fit for purpose’ you need three things: reliance, a D&B contractor, and a clear purpose.
You cannot include an obligation for something to be fit for a vague purpose or a purpose you did not tell your supplier/contractor.
In IBA v EMI (1980) the purpose of a TV mast purpose was to withstand foreseeable weather conditions for its location i.e. Emley Moor (which the mast did not do, and collapsed three years later).
In BSS Group v Makers (2011) the purpose of valves was to be compatible with specific branded adaptors (the valves proved incompatible).
In Lowe v Machell Joinery Ltd (2011) the purpose of a bespoke staircase “must surely include the compliance of the goods, when installed and used, with the Building Regulations.” As “…the goods, supplied in exact conformity with the contract, could not lawfully be used for their intended purpose, known to the seller, it does not seem to me that they were reasonably fit for purpose.”
In Trebor v ADT (2012) the supplier was not liable under a fitness for purpose term as no specific purpose had “been made clear… At no stage did the [buyers] ever indicate any particular purpose.”
If you want to rely on fitness for purpose, whether for individual components like valves, a £16,00 staircase, or a complete project, then you need to state what your purpose actually is.
Cases: Independent Broadcasting Authority v EMI Electronics and BICC Construction (1980) 14 BLR 1; BSS Group Plc v Makers (UK) Ltd (t/a Allied Services)  EWCA Civ 809; Lowe & Lowe v W. Machell Joinery Ltd  EWCA Civ 798; Trebor Bassett Holdings Ltd & Anor v ADT Fire and Security Plc  EWCA Civ 1158