Do whatever is best for the audio quality. There are always compromises to be made, and I would rather have them made in the area of battery life than in sound quality or power. Because you plan to sell it so inexpensively, people can buy those external batteries for it without going over a reasonable amount, if the life really is a problem. 10 hours is fine for me, as some of my DAPs only get 6-7. That is enough for a day or a long ride (I take breaks from listening, etc).
Replaceble batteries is an excellent idea James!