Get a small 1 to 2 amp charger that has a maintainer circuit built in. These units are designed to be attached to a battery and left connected. If you are going to have a automobile, you should have one of these. IMO, all cars should come with such a device built into the electronics of the vehicle so you could just plug it in. But, it would add $10-15 to the cost of a $30,000 item, so they leave them out.
Charging a severely discharged battery with the vehicle’s alternator is bad for the alternator and the battery. The charging rate is too high and the current draw on the alternator is higher than normal. Not that this will kill them immediately, but if you can wait and charge the battery with a battery charger, it would be best. One note, charging a dead or nearly dead battery will kill a brand new alternator.
Like I said, you really need a small charger (keep it in the trunk if this is your only vehicle) and use it to charge your battery if you can. If you have the car sit for more than a week or so, it would be best to charge it for a day or so before starting it. Particularly with today’s electronics in vehicles, a fully charged battery is a good thing.
Whether or not to pro-actively replace the battery? I wouldn’t. I would pay attention to the battery after being recharged and at the first sign of a problem, replace it, but if it is working fine, it will very likely come back with no noticeable damage. They don’t like to be deeply discharged and really don’t like being deeply discharged for days or weeks at a time, but I wouldn’t replace it if it was performing just fine.
excavating (for a mind)