1) MST vs. IAP... Ease of set-up would be a distinct advantage as well as cost. For examples, The MST will cost you half of what the IAP-175's will cost you. Both are outdoor rated. Also, the MST comes standard with an embedded 5 GHz antenna, the IAP-175, you have to buy them seperately.
2) iAP-104 vs. IAP-175... Indoor AP vs. outdoor. Functionally the same for your application as they run the same OS and are both dual radio. You won't be pushing transmit power or receiver sensitivity in your application, so no need to leverage the IAP-175 advantages there. You already have the main consideration covered: ensure antenna cable length is as short as 'reasonable'. That being said, we typically want to see the IAP-104 fielded for indoor applications and the 175 or MSR/MSR line outdoors. e.g. the specific tool for the specific job.