How about take:
Maths, Econ, Geo, Computing? and maybe add in physics as well?? or is 5 AS too much?? to be honest, if your mathematically minded (like you sound!) the computing, maths, physics should come naturally!
geo's quite an easy AS to take. all 5 would be good if you could put your mind to it, then if the pressure's too great, drop geo and another your enjoying less and concentrate on the three u like most.
Otherwise start all 5 and you could use the opportunity to get a feel for what the subjects will involve...you can drop one after a term if it's not for u!