Hello,
I was wondering on your post. If I were you, I was going to ask my Topcon and ESRI reseller/distributor about these questions. However, do you need this accuracy? Do you really know, that the advertised cm accuracy can be achieved with an external Geodetic antenna (probably with at least 10 min static measurement) and probably you need specific application(s) to enable/able to post-process and real-time corrections? These are the first questions that are just came in to my mind.
Another issue is to achieve the advertised accuracy in ArcPad, since this application itself doesn't support/compute/handle any corrections on your data on NMEA protocoll. Unless your GPS receiver does it internally and output those corrected data on NMEA, you won't get any corrected position in ArcPad without any specific extension - like GPScorrect for ArcPad from Trimble to handle real-time corrections for Trimble MGIS GPS-enabled handhelds.
So before you purchase, the more you ask, the more answers you got (or not) for your questions. And I really recommend to lookup on other - specific mobile GIS - forums to see whether it works or not. And if possible, ask product brochures and guidelines from your resellers and read them very thoroughly. (Usually, you can download them from manufacturers' website too, if your reseller doesn't provide them. Unfortunately, there are some of them, who tend to hide extra costs before you purchase, and it will be revealed when it's too late and spent a fortune on a device/system...)
Another good idea to ask a demo from your hardware reseller to see how the particular system (device+software) works. If they have nothing to hide, they will be ready to do this. Do not fear to ask, it's all about your money and not to mention, you should work effectively with the solution after all!
Hope this helps and won't be too late.
