Itineraries for Amboise using TGVs require a stop in Tours (St Pierre des Corps) and a train change to a TER train. This will be the case departing from either CDG or Montparnasse.
Departures from Austerlitz are all TER trains and most require changing trains in Orléans.
Purchasing tickets upon CDG arrival means paying the highest possible price with the risk of your desired train being sold out. However, as the mass vacation migration of August is over (until the return at the end of the month), you should not have an availability problem.
There is usually a 12h45 direct train from Austerlitz that arrives Amboise at 14h51. Cost is 36€. You could take a taxi from CDG to Austerlitz, fixed fare is 55€, and probably be in Amboise faster than taking one of the TGV options. You´ll not only save money, avoiding connections makes it easier with luggage.
Amboise is a small town and the train station is a good walk from most locations. You might want to arrange for station pickup in advance.