Welcome to the forum.
We will need some more info to try and assist you, but you will have to make the final call.
Where are you in the world?
How much do you want to spend?
Do you have any electrical experience?
Have you a PC to use if you need to?
Are you able/willing to retro fit mechanical/ electrical upgrades as you feel the need?
Cutting Ali on these machines can be done, but it can be tricky and alot of things need to correct in order to get a good result.
I've no experience with DSP's so cannot comment on how good/bad they are.
Mach 3 has been around for a long time so has a huge support base. it is also NOT being developed any more.
It looks a bit tired, but it works fine. Mach 4 is relatively new and is still being improved. Some machines don't have a 'pluggin' for them yet......
You have a steep learning curve in front of you. Take your time and enjoy the ride.