Transistor based bridges will not work properly at voltages less than 4V. CE drop will be too much and it will degrade efficiency.

L293D can work from 4.5V onwards. So, four pencil cells will do the job.

Max232 cannot be skipped if you want to avoid future mails saying "I don't know why serial interface doesn't works with my PC but it works with my friends PC."

Max4427 will not work, its low side driver. Also see the price on maxim's website 1.61$ each.

L293D is available for Rs50 @ cirkit electronics, lamington road.