MX300 is a slide-in replacement to the ARC RT-328, which also requires a G/S receiver for ILS. I'd imagine most ARC radio installations also include G/S capability and thus have an external receiver installed. My receiver was broken, but I found a working 2nd hand G/S receiver for $25.
May I respectfully point out, that your "price" for the transponder is way out of the norm. You can buy a digital GTX327 any day for $500, the prices at Bennett Avionics (where I think you got the prices from) are nowhere near realistic.
