When someone asks "What's best?", they usually get several different answers. What's best for one person may not be best for another. I'll give you my opinion, but it'll probably be different from others.
1) Look at Vectrics software, either V-Carve Pro, or Aspire. Without seeing what you want to do, I can't tell you which may meet your needs. www.vectric.com
2) Has nothing to do with software. See #4
3) Depends. Do you have the tools and skills to build one particular design, and maybe not another? Probably the easiest to build would be Joe's Hybrid 4x4. You can downsize it if you only need 24x48. You might also want to look at these plans. http://cncrouterparts.com/product_in...products_id=49
FUll kits are also available.
4) Motors need to work together with the entire system, including drives, power supply, and screws. Having said that, for a smaller machine, the best bang for the buck is the Gecko G540. Paired with a 48V power supply, and 380oz motors, it should run whatever you build just fine. You can get everything you need a www.cncrouterparts.com or www.kelinginc.net
Also, the dremel is a poor choice for a spindle. The minimum you should be looking at is a 2-1/4 HP hand router. Hitachi, Porter Cable, Bosch and Makita all make good ones. Next step up would be a 2.2KW water cooled chinese spindle from Ebay. Lots of members here are switching to them.
Before you make any decisions, I'd recommend spending a week or more reading and learning as much as you can here. Making your own informed decision is far better than doing what someone else tells you to do, which may or may not be the right choice in all situations.