Hi,
there are a number of very good motion control boards that work well with Mach3. I would avoid Chinese made controllers, if they work at all its usually with pirate software,
meaning that 'cheap' is about their only recommendation. Buy a European or US made board.
I use an Ethernet SmoothStepper ($180) made by Warp9TD. It has both a Mach3 and a Mach4 plugin and can therefore be used with either. I personally have been using Mach4 for seven years
and in my opinion light years ahead of Mach3. Warp9TD have been in the business of making, selling and supporting Mach ready motion control boards for nearly 15 years.
There are other contenders:
UC100, UC300,UC400 all by CNCDrive (beware the dozens of Chinese made UC100 rip-off boards on Ebay , Amazon etc......buy genuine or don't buy at all)
PoKeys 57CNC
CSMIO/P by CS Labs
Hicon Integra by Vital Systems
All of these manufacturers sell and most importantly support their customers, and while each has strengths and weaknesses ANY of them will work very well indeed.
Craig