Hi, Alana. It's nice to meet you. I think you're doing a wonderful job on being responsible by asking questions and taking everything into consideration before adopting. You've been given some wonderful advice and information already. I understand all too well about the allergy issue. When I was younger, I had reactions to cats. I couldn't be around long haired cats (my aunt had a Persian) without breaking out in a rash. I, also, couldn't be around Siamese (a friend of the family had Meezers) without my eyes swelling almost completely shut. Other cats could cause those reactions or could cause me to have the itchy watery eyes, sneezing, etc. Then when I was around 16, I found that I could be around some cats with no problems, but others would still cause a problem. Now, many years later, I live in a multi-cat household that includes long haired, Siamese, and others with no problems, so there is a chance that your body will change over time and you won't have to worry about allergies. In the meantime there are things you can do to assist with allergy problems. You'll find these things mentioned throughout the forum, but I'll let you know what I do. Even though, I don't have a problem anymore and neither does my husband or daughter, I have an aunt that's allergic to cats and my father-in-law is, as well, so I try to do things to make it easier for them when they visit. I keep the litter boxes very well cleaned. Rule of thumb is 1 litter box + 1 per cat. I have a HEPA air filter running almost all of the time. I vacuum the rooms with carpet regularly and keep the floors swept and cleaned where we have hardwood. I bathe my cats or rinse them down about once a month, plus brush/comb them a few times a week. We, also, make sure to change the air filter in our house once a month. These things seem to really help people that are allergic to cats be able to visit. During the summer months, we cook out with our neighbors fairly regularly and the husband is allergic to them, but he doesn't have any problems coming in our house.
As far as illness, some good points were made. Like Malyka...she appeared completely healthy at the shelter, but within a few days of coming home, she ended up with a URI, which required a round of antibiotics and constant care for awhile. Osiris was the same way. Siana was found as a stray and she appeared fine, but after close inspection, she had a wound on her neck and we went through several weeks of care due to weakness, diarrhea, and tapeworm treatment. These are just a few of the things we've dealt with recently. That doesn't count Teya having to go to the vet several times to get a scratch on her eye taken care of and a URI when we got her, then going through having her spayed and the healing process.
Good luck to you!!