Hi I can't see that buying an AGM battery will be any advantage to you in view of the cars age and assuming it is not equipped with stop start, the charging parameters are lower for the lead acid unit except when the temperature drops, then you have a problem as your existing set up generator may well rise to 15v, that will toast a very expensive AGM battery where as lead acid is far more capable of absorbing overcharge and still being serviceable, AGM will charge up to six times faster than a lead acid unit but won't tolerate overcharge, there lies the problem the charging systems on your car and newer are about the same but the BEM systems are different as the built for AGM system is highly sensitive and won't allow overcharge, I have also seen on this forum where one of the members bought an AGM to replace his old battery and caused himself all sorts of problems with weird faults, this is because you can't code an AGM unit to the older BEM so it can't work out why it can throw the worlds supply of voltage at the battery virtually unrestricted, this is because AGM has a much lower resistance to charge which is why it charges faster, in essence your $ will be much better invested in a top quality lead acid unit.
Steve.