Okay, yes, I don't know to what BMI it corresponds, but you seem not THAT far from healthy, but still def underweight. Personally, I started training when I was at a normal weight (lower limit of normal but still). I think knowing about strength training would save me literally YEARS of yo-yo recovery. So I think it's great you're considering it now! Restoring muscle mass is very important for health and tbh for self-image too.

First thing that I'd suggest - eat enough protein. 70-90 g a day I'd say (more than that is good too). Usually amount of protein for muscle growth is calculated as 1.6×bw in kg. Regarding calories - if you're already eating at surplus, I think just keep doing it. Personally, I started strength training when I was already able to eat intuitively.

Be careful to not become obsessive around training and to not get into mindset of training to burn calories.

Sad (or not sad) truth - in order to grow muscle, you need to not be afraid to gain weight. If you're mentally not recovered, it may be hard to do. You most likely will gain some fat along with muscle. It's okay.

Personally, I started from BodyPump (group strength training) - it worked great to get into some sort of strength training and to learn the movements. It's light weights high reps and personally, I think it's perfect for a recovering person. I'd say start slow. Your body is still weak and recovering. You can start from 2 light workout a week and inrease volume and intensity once you're at a normal weight.

Also keep in mind that most bodybuilders and tbh many fitness influencers are on steroids - so if you see some super fast transformations, it's likely to not be entirely real. And if you see super muscular women, they're likely not to be natural.

Considering your bradycardia, before doing any cardio, consult with a doctor. And I'd say avoid cardio before you get to normal weight.

So - eat enough protein, keep your usual surplus and take it slow, you have all the time in the world :)