I can not stress the value and importance in my opinion to you and anyone else considering building your own cnc router project to do a first build much smaller than 4'X4' I apologize for sounding like a broken record but I feel very strongly about this from seeing too many failed projects. I have NEVER had a problem selling any cnc router that I built for more than it cost me in parts (I would be disingenuous if I told you that I made a true 'profit' when factoring in my time) however, it is entirely realistic to expect and recoup the amount you invest in components so that you can reinvest in your 2nd / larger build.
In my experience though, if you build your small one correctly you will find that you will not want to part with it.
It is a mistake in my opinion to build your first machine at the size you are considering (4' X 4')...Not saying that it is impossible, just not the route I would recommend for a first build UNLESS you have someone that has been through the process a few times and is available and willing to assist you ON SITE not via email, etc. Just my opinion for what it is worth.
I would also encourage you to make a post on signs101 stating what you are looking for in regards to parts, old machinery to canabalize, etc.
here are more links I should have included in my earlier post:
http://www.cncroutersource.com/cnc-drive.html
I would call the above site a 'work in progress' which can be frustrating but good, clear beginner information.
http://www.cnczone.com/
simply a wealth of information
If the majority of the work you intend to do is metal obviously a plasma system is a solution, however, I would not recommend going this direction if the primary reason is cost reduction because you can manually control the 'Z' axis, this is an error in my opinion.
Build the machine that you need for the work that you do. If you are primarily a metalworking shop then disregard my prior comment , I am not familiar with the scope of work you produce.
An alternate way of obtaining parts is to contact an cnc service tech. They have a variety of odds and ends that can be used to build a 'frankenstein' machine for lack of another term... if budget is a major issue and you have the time to do the hunting this is an option and that relationship will be invaluable when constructing your machine and having someone to consult regarding the troubleshooting issues that will arise.
Do not limit yourself to the sign industry when looking for cnc parts, etc., as an industry we are a minor user of the technology.
For example I don't know of a cabinet shop that does not have at least one cnc router (Im sure they exist but I don't know of one) & the majority of them that I worked with when selling/servicing equip had multiple generations of machines if they have been in business for any length of time, all the way back to machines requiring manual entry of g-code.
Many of those machines were built to be true work horses with long service lifes but brands that for the most part are not recognized in the sign trade, such as Thermwood.
If you can find a dead, abandoned, or simply no longer used old machine (and they are out there if you have the time to search) you can scavenge high end parts...everything from the electronics ... to the table, there is a multitude of parts that can be reused and repurposed to meet your needs for a smaller machine.
I can not stress this enough! When you get to the point of planning the build of your 'real machine' not the 1st machine that I recommend, that you build a machine that does what you NEED it to do not what you can afford.
Save money, accumulate parts until you can build the machine that meets your needs versus sacrificing things such as size, or 3rd axis as you mentioned (which is absolutely necessary imo and not something that should be eliminated because of costs).
I do not know your ultimate goals but deciding what to build based on what you can afford in regards to a cnc router will be a decision that you will regret very soon after familiarizing yourself with the technology and it's capabilities (I understand financial "reality" but to choose the size of machine, basic capabilities soley on budget is going to frustrate you beyond belief with this type of equipment) just my opinion of course...
Build what you need to do the type of work you sell and "ideally"build your production machine large enough to accomodate the largest substrate you are going to regularly use (not just the table size but also the depth of the Y axis gantry).
Sacrificing 'luxuries' such as a tool changer (which is not a necessity in my opinion for 99% of router operators) is one thing, limiting yourself to a manual 3rd axis is a design/build decision that if you make you will regret quickly based on my experience, I hope that this does not come across as rude, that is not my intent, but this is a design decision that would be erroneous in my opinion.
Again to ANYONE considering undertaking the construction of a cnc router, it is entirely doable but PLEASE if you have no experience build a small machine first... something that will allow to route 12"x12" plaques or an 18"X24" sign to familiarize yourself with the technology and allow you to earn the experience necessary to build a quality machine that meets your greater goals without investing larger amounts of $ only to possibly discover that the project requires skillsets that you do not possess or do not wish to invest the amount of time required to learn (anyone can learn the skills necessary to build a cnc router, even if they have zero experience, however, most are not willing to invest the time to learn the various skillsets needed to accomplish the project).
This will allow you to 'test the waters' without investing the amount of time and money that is required to build a larger machine, I have simply seen too MANY people discover that it takes more time than they projected or than they are capable of investing or can justify to properly construct a large, production quality machine that is capable of acceptable, professional tolerances.
You will be able to identify all of the common pitfalls by building a small machine, which will save you much more time, money and troubleshooting, on your main build and ultimately allow you to build a higher quality machine.
I have simply seen too many people buy all the components intending to build a 4x8 machine and find it requires more time, money, and skills than they posess or are willing to invest and are left with a very expensive table to weed vinyl on or assemble signs on when they could have saved themselves thousands of dollars AND countless hours and most importantly they most likely could have completed their large build if they had gone through the process of a small builld first to gain the understanding of the system, components, etc. sorry to sound like a broken record I have just seen this play out TOO many times and know what it requires to properly build a quality machine and the vast majority of people who make the decision underestimate what it is going to take to have a successful ultimate outcome.
dan@esigns.com
Thank you for the heads up. 
