Do I need a datafeed for each Timed Evaluation?

No. During the Timed Evaluation the datafeed is included in the entry fee — you pay $0 for it. The included datafeed has calendar-month validity, so accounts purchased from the 25th of a month activate on the 1st of the following month. The Timed Evaluation's 30 days start at activation (the 1st), not at purchase — late-month buyers lose no evaluation time.