I've always been super scrawny and weak as piss. Starting strength barbell program (google it) and at least 5 but as many as 10 eggs in smoothies a day put 28lbs on me in about 8 months (which is a total of about 40lbs heavier than when I was a dumb sickly vegan about 7 years ago). I can't drink milk, which is probably a better/cheaper/easier way to put on weight. I could stand to lose about 5-10lbs of fat now, but since I'm normally skinny and have a small appetite, it will come off pretty quick now that I've recovered from surgery and can ride my bike again.
10 eggs a day gasp!! Well, I've been tested for cholesterol and I'm in the top 4% of the country, so don't worry about eating lots of raw eggs. I'm told by my doctor, and read it elsewhere myself, that grains are a bigger contributor to high cholesterol and cooking the egg yolk is a no-no as well.
Benefits? Fixed my long term backpain. 15 years of pain and now I have none, and can sleep in any position I choose and not wake up in pain. That's the best benefit. But also feel way stronger in everything I do. Sprinting a bike has improved greatly. Distance stuff is much the same. Picking up fallen logs or groceries is massively easier. My posture has gone from brutally shitty to the way it should be. Had to buy a bunch of new clothes.
It's not easy. You have to lift heavy while you eat or you will get fat. You have to eat a lot when you are the opposite of hungry. You are probably light because you don't have much of an appetite, so stuffing yourself again when you're already sick from your last meal 4 hours ago is as hard as squatting til your eyes bulge 3 days a week. But even though I've been forced to back right off due to an injury requiring surgery, now that I'm back on it I've kept most of my strength and weight so I've created a 'new normal' for my body.
You say you want upper body strength mostly. But don't be fooled into neglecting the most important lifts. If you want to gain weight and get strong everywhere, you need to squat and deadlift as a first priority. There is no 'leg day'. Every workout is leg day. Squat, and then worry about what comes next or you will be asking the same questions another year from now. Good luck!