Murcia San Javier airport is poorly served by public transport. Last year the only bus in the evening left approx 1 hr. after the Ryanair flight from Dublin landed. This may have changed now . The buses in Murcia are run by a company called "Latbus" ? try googling them for timetable.
There is a bus runs from Murcia to Archena , although not neccessarily drop off convenient to Spa Valley development, it is too far to walk out from the town to the development if you are carrying bags - OK if you are fit and not lumbered down . Either Bus no. 42 or 142 serves Archena from Murcia Bus Station .
Car hire is the best option, the roads are good and relatively straightforward to get from airport to Spa Valley Archena. Allow one hour for first time.
Do not neccessarily go for the cheapest internet option . There may be extras payable locally. Check out what is and is not included. Finally dont take the cheapest smaller cars as you will need some power on the motorways. Also the boot space is limited . Consider Alicante airport as you will have options other than Ryanair. Alicante is 45 mins drive to Murcia and a further 15/20 mins out the Madrid road will bring you to Archena Spa Valley.
I stayed in the Archybal Touristic Apts . New clean spacious and cheap. Accom only . No pool or food but a few cafes on the same street.
If you want full service try the Hyltor Hotel just down the street . Full Pool & Spa facilities. If you want to be actually in the Balneario or Spa and have the full treatment and relaxation, stay in one of the three hotels that are on site but they are expensive and food just OK.. Alternatively rent an apartment in Spa Valley Phase 1. I can give you a name and Tel No. of the chap who administers the rentals there.