Are you looking at more of a carvewrite size or larger?
If larger, as Carol mentioned, there is additional equipment such as dust collection, vacuum hold downs, bits, software, etc. Some of that equipment you may already have and use for other equipment/processes. I think you have to determine what you're going to be building and if the items will pay for the equipment over time or if you are willing to eat the cost if it just makes your life easier. Also look at how often you're going to do it, there may be a family member here that can be used for outsourcing some parts, such as laser carving/cutting.
I've built my own CNC and have been collecting parts to build a larger one. You spend a lot of time tweaking, fine tuning, and working around short comings, but I think we do that with any tools in our collection, the cnc is no different. Depending on your budget a turnkey solution may be more suited to your needs, like I said, you have to determine what you're going to use it for.
To a small child, the perfect granddad is unafraid of big dogs and fierce storms but absolutely terrified of the word “boo.” – Robert Brault