Yes you "MIGHT" need a new BOB to use Mach3 - but maybe not. I don't know. There are just so many configurations out there that is is impossible to know how to fix your problem. So, if you are not getting the answers you want to hear, it is not because anyone is withholding information. It is because nobody really knows the answer. All anybody can do is to recommend what they know works for them.
Mach3 has been around for a long long time and has proven itself many times over. I see very few people posting about difficulties in using Mach3. On the other hand, I see many post about the difficulty of using something like GRBL - or Garble, as I cal it.
I had trouble with my commercial machine for a few years. I was VERY frustrated with it and gave up on 3D. I did buy, Mach3 and a reputable CNC4PC BOB. I am not good at installation of that stuff so I bartered for installation. POOF - my problems went away and here I am 4 years later - never looked back.
As a rule, I look for the user friendly way out in most everything I do. Vectric, Mach3, and so on - fit the bill.
The posters here ARE in fact pointing you in this direction. So am I.
I do understand your desire to keep cost at a minimum.
I am NOT well versed in OX or Inventables
As to the OX and Inventables. There ARE people that make those machines work - so it is do-able. Perhaps on the inventables forum, of user groups, or tech support will be a better source of help.
Imagine the Possibilities of a Creative mind