You meed to know the quad before you can know the battery choices, the size (in MAH, Milli Amp Hours), number of cells (1s thru about 6s as they are called), and specific connector are all decided by the quad itself. If you have a bunch of 5" quads, then you can usually run the same batteries on all of them, but you cant use a 5" qyuad battery on a Whoop class quad for instance.
A very discharged battery (3.2v to 3.4v) can take up to an hour and a half (or more) to charge safely at 1C (1 x Chargerate), most of the time we storage charge when we get home so our batteries will be or recover to (30 minutes to cool and stabilize) around (3.7v-3.8v) so it will only take about 45 minutes to an hour to charge.
Chargers are also a question best answered after you have an idea of what you need at first, a decent charger and additional cables and boards you will want will be more expensive than a cheap one that can only do 1 battery at a time, who wants to spend 10 hours babysitting the charger?
Last but not least... YES, batteries of the Lithium Polymer variety can be very dangerous, and much like a jealous girlfriend, any perceived neglect and they will burn your house down
Batteries are most dangerous when they are fully charged, being charged, extremely discharged, and when crashed into things during flight, but there are ways to mitigate most of them, the rest is up to you to be vigilant with them. That being said, very few people have batteries go on them, but you do not want to be out of the room if the battery and abuse gods pick you this time.
This is a fairly complex hobby, and things don't always work like they should, but it is a hobby for tinkerers and most who "just want to fly" end up selling all there stuff and getting out because they didn't have the patience to learn it. Do your research, ask us a ton of questions, and we will give you the benefit of our experience so you make fewer mistakes.