@Crow Jane glad I could help! I'll try to help with your other questions as well, and don't hesitate if you have any more!
1) Since I've never had male rats, unfortunately I cannot give you advice based on experience. However, I can help based on what I have read online. Mammary tumors are pretty common in females, and the most common tumor to get. It is possible for rats, male or female, to get other types of tumors however, like the more deadly pituitary tumor. Additionally, it is possible for a male to get a mammary tumor, because it's just like how breast cancer in humans mostly affects women, it can also affect men. It is much more unlikely, however. I was pretty close to switching to male rats just to not deal with the heart break of a female with a tumor, however just based off of personal preference I decided to go with females. No rat is exempt from tumors unfortunately, but if you want a better chance of avoiding mammary tumors you can spay a female rat, or get a male rat.
If your rat develops a tumor, you do not have to have surgery to remove it, as long as the rat is happy and able to enjoy life. Since rats usually get tumors when they are elderly, it can be too risky to even put them through anesthesia, and if they survive that the recovery will be slow and laboured. This is something that would need to be assessed by a professional vet in person. Additionally, even if the tumor is removed, it is possible for more to appear. Therefore you'd need to consider the rat's age and ability to recover, as well as the likelihood of more tumors appearing.
You can improve your chances of avoiding tumors by getting your female spayed, or getting a male whose likelihood of getting mammary tumors is greatly decreased. A much easier option, however, is to give your rat a strict and healthy diet, and any time you want to give your rat a treat just make sure it is healthy. Rats love fruits and veggies, which are a healthy alternative to sweet store bought pet treats. However, keep in mind that fruits are quite sugary and should be given in moderation. I personally feed my rats Oxbow Essentials, which was recommended to me by multiple vets. Occasionally they get rice puffs, oats, and chicken as a very special treat c: I know everyone wants to spoil their little ratty with tasty food all the time, I would love to as well! But I know that the best way to show them my love is to keep them healthy, rather than give them unhealthy treats all the time.
Other than tumors, rats can get respiratory issues. All rats are born with a respiratory disease. However, it will most likely stay dormant and never arise nor cause any issues. It can happen, however, that the disease does surface and the rat will have laboured breathing, wheezing, and sneezing. It can be easily treated with antibiotics. To avoid respiratory problems, make sure their bedding is dust free. Many companies label their product as "dust free" however if you shake out the bag you will see tons of dust come out. When you find a bedding you'd like to purchase, check online for customer reviews from pet owners who have actually tried the bedding, and see what they say before you buy it.
Overall I would say that males and females have about the same health risks, other than mammary tumors.
2) Yes, exactly that! Keep a close eye on your cables and check them often for any chew marks. Personally, not a single rat I've ever had chewed or showed any interest in the one cable I have in my room. I'm not sure if that's pure luck or if they just don't really care about it. So I would recommend for the first few days to inspect your cables and make sure the rats are not chewing it. If any are plugged in, make sure they are secure and fully plugged, with no exposed metal. Make sure you have no plants in the room. Some people say as long as it's not poisonous, a plant is okay, however I personally disagree. Plants require soil which has tons and tons of bacteria from outside. Because rats are rarely vaccinated, it's important to keep harmful bacteria away from your rats. I'm not saying you need to wash your clothes and have a shower every time you want to play with your rats, but I am saying that since the soil has lots of outdoor bacteria, tiny bugs, and manure, I would keep it away. Whenever I need to take my rats to the vet, the first question they ask is have they been outside. The same way that indoor cats don't need the same vaccinations as outdoor cats, or a puppy that's too young to be vaccinated, rats should be kept off of outdoor grass and soil.
Another part of rat-proofing a room is not just making it safe, but also making sure they don't chew something they're not supposed to. Some rats don't chew your things, and some do. For example my first pair of rats never chewed up my things and in fact weren't very interested in the chew toys I gave them. However, the rats I have right now are more mischievous and will chew a book if it's laying on the floor, and they love their chew toys. Interestingly, I have a book shelf in that room and they don't chew any books that are properly stored. However they'll munch on any on the floor! Maybe they're telling me to clean up haha! So it really depends on the rat. It's very easy to just put things out of reach for when you have them out.
3) It depends on the vet clinic, since every one will be different. From personal experience, just a check up alone used to be $60 Canadian, but recently increased to $80. Treatment wise, the most common medication given is simple antibiotics, which I paid $20 for over 14 doses. Spaying can range quite a bit so it would help to ask your local clinic. Even if you don't have a rat, don't be shy to call your vet clinic and ask! Since perhaps the cost will affect your decision on males or females. So long story short, you can in fact spend hundreds in order to keep multiple rats healthy.
@zach herron. I'm so happy to hear that! Parents can be hard to convince, but the first time I asked for rats my mom said NO, NEVER and I thought I'd just have to wait until I moved out. It took time for her to get used to the idea and when I realized we'd need 2, not just 1, she said no way. Luckily she eventually approved because she saw how badly I wanted them! When we first got them, she wasn't very interested in them. But now, after many happy years with all different rats, she loves them and cuddles them and baby talks to them! Hopefully your parents might consider this as well <3 If not, rats are still definitely worth having one day!