I don't have a Sherline, but I do have one of their rotary tables, CNC version, that I use on my Taig CNC mill. The Sherline has a great rep for a quality product, but like the Taig, it is a light duty mill. Light cuts and small tools are a must. There are a number of Yahoo Groups that will help you with the setup and use. Basically what you need is a CAD program. There are a number of them. Some free and some expensive. Then you need one or more CAM programs. SheetCam is a great, and not very costly. It is great for 2D and 2 1/2D work. About all you need for modeling unless you are going into 3D cowls and such. MeshCam is a good 3D program. There are a bunch of others.
Then you need a G-code to machine program. Mach3 is kind of the gold standard for both hobby work and industrial work. Again, not very expensive. There is also TurboCNC, a Dos based package. These will take your CAD drawing to a finished product.
For the tooling and other items, check out Use-enco.com They have very good and not expensive tooling. There are other suppliers, like Shars, The Little Macine Shop, A2Z and others can provide you with tooling and work holding tools you need. For Raw Materials, again Use-Enco.com and McMaster-Carr are good choices.
Now, for the missing pieces, you need to get on a Sherline group on Yahoo Groups and get some info. If it is a home built conversion like my Taig is, the there are Groups like DIY-CNC that can help on getting you up and running.
The options are many as well as are the cost. There isn't a really good, cheap, and usable fit all package. Just remember that if the product is free, then you are the product they are selling. Some exceptions, but not many.
Check out the "missing Serial" cable. Most likely it is a Parallel Port cable. There are hidden pitfalls there also. Some newer PC's, Lap tops for sure that have voltages on the LPT ports the are in the 3.5V lever vs the 5V of the older ports. Some of the CNC controllers need the 5V signal levels. You need to find out what your "driver box " needs and plan from there. There are other pieces of hardware to bridge this gap. IE BOB’s Break out boards, some of which will “repower” the signals. Lots of options.
I've thrown up a lot of things to look out for, most all I have stumbled over in the last 5 years I've been playing with CNC milling. These are not to scare you off your project, but to point out the pot holes you may encounter. It's always better to have someone else have the problems and you learn from them. The check book likes that.
Welcome to the frustrating and rewarding world of CNC. Hope this give you some ideas.