You can actually do this multiple ways. Lucky for you the requirement is for only reading data so it should be pretty simple.
You have a couple options.
1. Create a brand new webservice and add x++ logic to return the information you need. An example can be found here axcalated.blogspot.com/.../c-creating-custom-ax-aif-web-service.html
2. Create an aot query object and access it via one of the built in ax webservices called queryservice. An example of this way can be found here axcalated.blogspot.com/.../c-creating-aif-webservice-to-access-ax.html
3. Create the query parms on the fly within the outside application to be called/passed to the queryservice & metadataservice axcalated.blogspot.com/.../c-ax-aif-webservice-apply-company.html
4. You can also use proxy objects within .net and an example of that is here axcalated.blogspot.com/.../accessing-ax-custtable-dirpatytable.html
Personally I would recommend #2 if you need to return a bunch of fields from different tables. But #1 if you just want to return 1 value, unless said value is a field on a table then it would be #2 again. I would recommend #2 because its a built in web service so no need to recreating the wheel (unless you need to) and the most documentation is available for this method.
In order for you to make your decision I think you need ask your customer what data they actually want returned. One field vs multiple. If the information being pulled from tables structures or logic based?
Hope this helps