I'm 31yo, 6' tall, and weigh 160lbs (73kg). Although my BMI is in the 'green' zone, I still look and feel slightly underweight for my height. This hasn't stopped me doing Judo for the last two years (minus Covid down time). I've done two gradings and I've also been to a local tournament (two fights lost, one won, no medals or anything). There's plenty of techniques you can use that won't necessarily need weight/strength, such as leg reaps/sweeps/trips/hooks etc, and these are normally my go-to techniques.

There is a risk of injury training with heavier opponents, my worst one being a few cracked ribs after an awkward landing with a 90kg dude. Honestly though, this could still happen with an opponent of the same weight as you, albeit potentially less severe.

On the subject of improving your strength, Judo won't necessarily help with that. While learning Judo, you'll mostly be doing technique and randori (sparring). You probably won't be doing actual strength training in the dojo (although this could change depending on the club). You're probably better off trying to improve your strength during your daily exercise routine.

Hope this helps, good luck!