Can certainly help but unfortunatly it's not so simple has just saying buy this and that.!!

We need to know some more about the machine and your plan to turn it into cnc capable.? IE: Do you plan to use the existing screws etc and if so how good are they.?

There are so many variables that untill more is known about the machine it's self and your idea's to make it fit for CNC useage then the route to take can't be fully advised upon.

Don't let me saying this put you off or discourage because it's certainly more than do-able but there are certain things on the machine that need to be right otherwise the potential for disappointment is huge, not to mention wasting money.! . . And thats without buying the wrong components which like you correctly worked out is very easily done and unfortunately so often happens.!

Software wise to do this CNC millarky you need, in the main a few key softwares.??

1st Control software to command the machine and interprit the G-code and perform other functions like monitoring safety, turning things on/off coolant, dust extraction etc.
The 2 main softwares used in DIY Cnc are Mach 3 for windows or EMC for linux. Both are very good, EMC is free but to get the best from it it helps if you speak Geek.
Mach 3 is by far the most often used by DIY'ers and has a huge wealth of experienced users to call upon if needs be, it's also the one I use and prefer so I'm obviously slightly biased. .Lol

Next is software to create the tool paths, better known has CAM software. In Cam you take the artwork or model and from it produce toolpaths which you then run thru a separate but combined portion of the Cam software known has a post processor. This produces a G-code file that the control software (Mach 3)can then act upon and follow it's instructions.
It's within Cam that you select things like cutter size and the speed and direction at which the machine will travel (feed rate) amongst other things.

The artwork can be created in CAD then imported into CAM or there are available CAD/CAM packages that combine the two in one package. They both have there merit depending on the jobs but in the main I find the best and most flexable being CAD to do the artwork due to having better tools so quicker creation, Esp for 3D models, and use Cam just to create toolpaths/G-code.
For simple stuff like text engraving or 2D work like cutting profile shapes, pockets etc then I often do this straight in CAD/CAM to save time.

The software you choose will depend largely on the work you do and the size of your wallet. It can be done for virtually free thou often with large to massive lurning curve.
For a first time users I suggest you look to the vectric CAD/CAM type softwares or for not a lot of money give CamBam a look, both offer free limited use so worth down loading, same goes for Mach 3.
Just remember has lots of folks new to cnc misunderstand control software like Mach 3 and most other control softwares do nothing but control the machine.!

Hope this helps.!