Pete,
I would download the lifeline technical manual, you can find it on their site under knowledge center. Then I would look for a charger that lets you set their charger states or chose settings that match the values in the the lifeline document (absorption 14.3, float 13.3) @ 77°F.
Many chargers give you multiple battery type settings and you get to choose the one that most closely matches your battery. Some go a step farther, and allow you program in the values for these settings.
I would also recommend one with temperature compensation probe,
I don't have a stand alone charger as mine is charger/inverter model (Magnum Energy) .
I have heard that the Xantrex Truecharge 2 is a pretty good unit, but I have no usage myself. its AGM settings do match up well with lifeline.
Xantrex TRUEcharge2
40 Amp $339.00 40 Multiple Batteries, prioritize house, secondary battery charge
60 Amp $545.00 80 Multiple Batteries, prioritize house, secondary battery charge
Options
Remote Panel $79.00
Battery Temp Sensor $35.00
a cheaper alternative is
KISAE Abso
40 Amp $259.00 10 -40 A Multiple Batteries, prioritize house, secondary battery charge
60 Amp $329.00 10 -80 A Multiple Batteries, prioritize house, secondary battery charge
Options
Remote Panel $59.00
Battery Temp Sensor $16.95
Both of these are multiple battery chargers, meaning you could also hook up to starter if you wanted.
-greg