The previous Tata Sierra which was discontinued was a 7-seater car but as of now, the brand hasn't revealed any specification of the upcoming electric SUV Tata Sierra. So we would suggest you wait for its launch and stay tuned for further updates.
It would be difficult to give any verdict because so far Tata hasn't stated that the Sierra will come in production or not. If Tata finds the SUV commercially viable, it will be put into production in the near future. So we would suggest you to wait for its official announcement.
The Tata Sierra EV is likely to feature a further developed version of the Ziptron EV powertrain for a range of around 400km on a single charge. The production-spec model may arrive with petrol and diesel iterations before the EV. Moreover, there is no official update from the brand's end regarding the transmission. Stay tuned for further updates.