Welcome to the Zone.
I can only offer limited advice, as I am a newbie & have not built a machine. So take my advice for what it is, a newbies view.
As far as the Dremel is concerned, it will cut acrylic, wood and maybe even drill holes in aluminum. A small trim router may be a better choice, depends upon YOUR needs. IMHO, shorter (less travel) for the Z-axis is usually better (more rigid). Do you really NEED 15" of travel? Many things determine the machines capabilities. The thing is that ALL machines are a series of compromises, speed Vs. accuracy, cost Vs. size/rigidity, design Vs. ability to build such design, etc. Many have built MDF and aluminum extrusion based machines (that meet THEIR needs), however, every design and material choice affects overall machine performance
characteristics. You first need to determine what you want you machine to be capable of doing and how well you want it to do it. Characteristics you need to consider/determine: 1) Speed of machine, feed rate, jog speed 2) What tolerances are acceptable, (accuracy & precision) 3) Budget constraints/parts availability 4) Tools available & your skill level for the build These are some of the things you must consider prior to designing YOUR machine. My advice is to do your "Homework". You have come to the right/best place to do that homework, the tutors (members) here are great. Here at the Zone, the true "Experts" are willing to answer your questions, but you really should do your homework (research the zone and learn)first. Just another newbies uneducated 
insights. Best of luck. Randy,