Is Booking.com a reputable organisation? - I would definitely say so. I work in a hotel and we use them for the majority of our bookings. I have also used booking.com many times myself. I have booked to stay at The Horizon in October through them.
You do need to submit your debit/credit card details but that is only to secure the booking. You usually have up to a week/few days (depending on the hotels policy) to cancel your booking without any charge. I cancelled my original booking for October to book with Horizon, there's no hidden catches. So long as you make any amendments before the stated times/dates, there is no charge. I think most hotels take payment in person once you arrive and you can pay either in local currency or by debit or credit card.
As for the taxi to Ipsos, it's probably going to work out more expensive than the hotel room price. Going by a fair few reviews on TripAdvisor, (recent reviews as well) it seems that they don't really distinguish between a 'taxi' and a 'transfer'. There seems to be a standard charge of around €30/€35! which is rather expensive for such a short trip. It does say that there are regular buses that only cost around €3!
Rachel x