Building your own doesn't necessarily mean you have to assemble it on your own; it just means you have to choose the parts. A local computer parts retailer will certainly give you advice on what you should buy and get all of the parts in and assemble it for you, all for the price of buying the parts separately.

Don't underestimate the saving you'll make here: I built my computer for $NZ1600 instead of $NZ3000 for an equivalent machine from a chain store. That's nearly a 50% saving.