You won't have any problems getting a seat on either the 0546 (arr 0838) or the fast 0651 (arr 0900).
I can't comment on punctuality, but I know that there was always pressure getting the 0651 on-time when leaving Taunton, otherwise it could clash with stoppers/freight around Westbury which could delay it as it has zero slack in the timings. If you need guaranteed 0900 arrival every day, then the 0546 would probably be the way to go, but obviously an early start
I'm not sure about exact Car Parking charges, there are 3 car parks operated by Apcoa at
EXD» , the further of the 3 car parks from the Station (by the level crossing) is the cheapest. Over the level crossing there is a car park on the left which is privately owned and offer cheaper rates/season tickets I think.