The key is to buy an 'IT solution' rather than just a collection of hardware and software.
If you enter into a contract to purchase an assembly of various hardware and software items, that is what you will get. Whether it does what you want it to will be your problem.
Instead, you can contract to purchase an IT solution. Rather than just specifying what items you want to purchase, you also specify what you want the system to do. For example, if you want to network some computers in your office, instead of just saying that you want specified software and cabling, you specify how you want the network to perform. The supplier will then also be responsible for meeting that 'functional specification'.
