0525 arrival should be fine for 0645 flight, but the bus stops are a convoluted walk to/from the Hilton and I wouldn't really want to walk or wait in the cold at 5am in February with the risk of missing the bus by a minute if I woke up late
Even if you download the app and buy an open return ticket that would be £14 each, so average £14 each way if there are two of you. The Hilton website says a taxi is £15
If you want to save money on the first trip buy a combined bus/rail airport ticket from the 757 bus driver and go to Anderston station, which is also a convoluted walk to the Hilton