If you want to contain costs then 4,5,6 aren't required:

4) In 2) you only mention needing 2.5D, in which case you can just use the free version of cambam since it does that just fine.
5) Try LinuxCNC, contrary to popular belief it's not difficult to set up and is completely free with plenty of features. If you have problems with either Mach3 or LinuxCNC, be it in general use of just setting up, you will easily get help on this forum. Once you've tried both programs decide weather it's worth about £100 for the Mach3 licence.
6) See 5).