Greetings fellow Apple Certification seeker! I have a hard time finding other people within this forum that are actively seeking their Apple Certifications. (I'm one!)
To you question, it is really hard not to have at least two machines and do the exercises in the PeachPit book. You really do need at least two Apple devices. And furthermore, if you want to avoid messing with your current home use set up, you really need to install the test framework on external drives or at least partition your current devices with about 128Gb (you could get by with 64Gb) of space and install the test (pretendco.com) environment onto those partitions. You can run Server within a wireless environment with no problems and complete pretty much all the labs for Support Essentials with no concerns. Just give it a static address though.
However, if/when you go down the road for the Server Essentials exam, you will be making changes to the server box that are in some instances, not easily backed out of. In fact, the best thing to do once you have completed those labs is to delete and reinstall OS X on the device, or just delete it if you never plan on using it again. Too many internal server configurations are just not easily backed out from.
I have a Mac Mini and a MacBook Pro that I have been using for my certification training. I have an external T-bolt LaCie drive that I have partitioned and use that for my OS X Server environment. As for the MacBook, I image the day-to-day OS and install my test OS onto the MacBook and then I just blow that environment away after I am done with my labs and reinstall the day-to-day image back onto the MacBook.
Yeah, it takes time and is sometimes a pain. But it also works for me just fine. Oh, and btw, no, you cannot use your iPad as an administering machine, that simply will not work for you.
You can pick up a Mac Mini 2011, for a fair price if you keep an eye out for one and use that in you lab environment. Even the 2012's are finally starting to drop in price that makes them very reasonable for a test lab environment. All depends on what your budget looks like for you.
If all of the above is not gonna work for you, you can do the exercises in the PeachPit book up to a point, just do not reconfigure you device when the steps call for it. You can simulate a good deal of the labs, you just will not get the full benefit of each lab.
Heads-up. If you have not heard yet, Apple has decided to delete the Server Essential (ACTC) certification as well as delete the MAC Management exam and is just going to have two certifications for El Capitan.
Apple Certified Associate (ACA) MAC Integration 10.11 and
Apple Certified Support Professional (ACSP) 10.11
Here is the link to their site with this information.
http://training.apple.com/certification/osxyosemite