I gelded my 6 year old gorgeous leopard Appaloosa stallion after he recovered from EPM in order to make his life and consequently mine, less stressful. He had HUGE testicles. My vet did it in my stable yard and it was uneventful. However, he did have an infection post gelding which took a couple of months to clear. I think it was induced in part by the generous loose skin he retained after losing his melons. He had only been used for breeding on a limited basis. In addition to hand breeding him, which he was very good about, I had tried allowing him to pasture breed and then remain with the mares once bred, but he was far to aggressive and pushy in that setting, and was removed. He was something of a hard head as a stallion and is certainly still a hard headed, stubborn guy as a gelding.
While perfectly fine to handle under saddle and on the ground, from a pasturing/turnout standpoint, he remains agressive and an alpha male, and you have to be careful about who you pasture him with. He will be top dog; he can be possessive about mares and he is not to be trusted with younger colts and geldings. He attacked and savaged a 8 month old colt I owned after breaking out of a stall and going after him. Fortunately the colt did recover but was left with multiple areas of dimpling in his neck from crushing bites sustained during the attack.
I did have success allowing him one companion at a time in a pasture of his own. One was a Welsh cross pony gelding and the other was a mini jenny donkey oddly enough. Both were quick to get out of his way though when he was in a mood, particularly at feeding times, which was a prerequisite.
He is currently on loan to a friend of mine with a therapeutic riding center. At 16hds and quite stocky, and most importantly, extremely lazy, he is a good, solid mount for some of their large adult riders. He is now in a coed pasture of mature horses, but he is definitely the boss. He no longer acts at all studdish, but he is definitely going to be the top dog of any grouping when it comes to pasturing. He can be trail ridden and trailered in mix company just fine though.
As others have said, a lot does depend on how he is now. Gelding him will remove the libido, but if he’s kind of a jerk now, I strongly suspect, he will still be kind of a jerk post castration. It will however definitely improve his quality of life, you just may need to ease him into it and then keep an eye on him for a while and pick his pasture companions carefully, and/or warn any potential buyers.