Hi, welcome to the forum, glad you joined.
I think getting a rooster will be a good idea but should it be older, baby and should I wait till spring so it can be quarantined first and introduced slowly?
People do this all sorts of ways, we all have our own preferences. My goals are different to yours and my situation is different. For you I'd suggest one of two things, mainly based on your thoughts on biosecurity.
I consider eggs that you hatch or baby chicks from an established hatchery to be the safest from a biosecurity respect. But you have to wait seven or eight months before they grow enough to be of much use. Raising them and integrating them can have issues.
I don't know how much you know about how to do a proper quarantine and the risks from that. Many people on this forum don't do a good job, we are just not set up for it, let alone have the knowledge of the details. I find the easiest integration is just turning a mature loose with a flock of mature hens. My pullets start acting like mature hens about the time they start to lay. Your young ones ay be a bit young for that.
I have 6 hens 2 are 8 months old and 4 are 5 months old. They work well together and are very sweet. Not as snuggly as I wanted it of my chicken family but sweet nonetheless.
Until my pullets start to lay they tend to avoid the adults. Not always but usually. If they are not laying yet they should become more of a happy family when they do.
I love in the country and have recently felt the loss from the predators in the area.
What predators? I find roosters to be pretty worthless against larger ground based predators like coyotes, foxes, dogs, and bobcat. I'm not sure where you are located so I don't know what predators you may have. Most of these are ambush predators and don't leave any time for a warning before a chicken is taken. I've had dog and fox attacks. My roosters are much more likely to try to lead the flock to safety instead of fighting a rear guard action. I've never had a rooster lose a feather due to an attack on his flock.
One thing a rooster should do is if something suspicious is going on he will get between whatever it is and his flock and kind of check it out or at last give a warning. This probably makes him the primary target if something is going on. So some benefit to that.
A good rooster should be on the alert for flying predators. Often the dominant hen will take on that job if no rooster is present but a good rooster should be an early warning system for that. Again mine tend to lead the flock to safety once a threat has been identified, but sometimes he will attack a hawk if it can, say the hawk has a hen trapped on the ground.
These are my opinions, others feel differently.
Should I let them out to free range again or keep them locked up?
I don't know what happened, what predator, or how risk adverse you are. I don't know what your facilities look like. If you can, I'd suggest you keep them safely locked up for three or four weeks as a minimum, give the predator the idea that a free easy meal isn't necessarily available for the taking. Sometimes that helps.
Any time you free range they are at risk, even if you are there with them. People on this forum have had a fox or hawk take a chicken just 10 to 15 feet from where they were. I permanently remove predators that are hunting my area, that removes one immediate threat but there can be others that can move in if they are not there already. It's not unusual for me to trap a different predator two or three nights in a row, there are that many already there. New predators are being born all the time and they need to find their own hunting range.
The best defense are good barriers, depending in which predators you are defending against. I use electric netting which has totally stopped all ground based predators. Flying predators can still attack but I haven't had any real issues with those. Others are wiped out almost immediately by flying predators. We are all in different circumstances and get different results.
Whether or not you let them fee range again is your personal decision, I can't make that for you. I can't tell you how big the risk is, you may be able to go for years without another attack or you might have one immediately.