i'll answer these questions right off then send you to a couple of my other posts about this relative subject that may answer some more questions.
1. males do mark and some say more then females. in my personal experience it really all depends on the personailty of the rat. all rats will mark a little bit. boy's marking does tend to have a bit more odour to it and ammount depends on the size, age and personality of the rat. some rats will not mark at all and others will claim eerything is theirs at least a few times.
males get larger then females but i find they are a good size. you're not going to find a rat over 2lbs no matter how fat it is. females tend to be under a pound and males tend to be slightly over when full grown though there are exceptions to everything you're not going to find a fancy rat over 2 pounds. if you get a gambian pouch rat (which are not sold in most petstores) you could expect the average size to be 3-5 pounds and they live for about 7 years if properly cared for, but they're hard to find and not really the type of rat you're likely to be talking about.
rats can get tumors at any age but that doesn't mean they are going to get them no matter what. females tend to get them more often then males and generally they don't show until around 2 years. not all tumors are concerous though its always good to check. males can get tumors too but females are 78% more likely to develop them. this is mostly because females have estrogen and have breasts (nipples) while the males don't as the most common tumor is breat tumors. on that note human females have a higher chance of developing tumors and cancer then human males as well though i don't think the percentage is quite so drastic, though i could be wrong. the speed that tumors grow will vary from rat to rat and different types. there are some tumors that a rat cna deal with for a time on their own (until it gets too heavy or incumbersome) without any special treatment and their are others that will need to be removed imediately and still others that won't present until they're fatal, it all depends on the placement and if they're cancerous or not.
medical bills will vary from vet to vet and illness to illness. its not uncommon to shuck out hundreds of dollars trying to catch up with a respiratory infection though this is not always the case. the important thing is to find a rat friendly vet. they don't have to be an exotic vet (as they tend to boost their prices just because they're considered exotic) but they do need to know a little bit about rat health or be willing to do the reasearch to meet your needs should your rat get sick.
neither males or females tend to really fight. they will have dominice squables which is them just pushing each other around and sitting on them really but the general rule is, no blood no foul. get ones that were raised together from a breeder or accidental litter if you can as i take it this is your first pet rat experience and they tend to be better socailized. a rescue is another great place to get your first rats. i know of some rescues in canada and i'm sure you could find tons in the states. i know there's a few in europe too that are very good but i don't know where they are. i don't know about the rest of the world.
now i will send you to the other threads. they should answer some other questions you might have and if they don't answer them all the people here are all willing to answer more of them, especailly myself.
http://www.ratforum.com/Forums/viewtopic/t=336.html - getting girls or boys
http://www.ratforum.com/Forums/viewtopic/t=281.html - how to do socialization if you need it
http://www.ratforum.com/Forums/viewtopic/t=292.html - a bit on nutrition
http://www.ratforum.com/Forums/viewtopic/t=238.html - introducing strange rats for problem introductions and some on the importance of proper cage size
i have also written a booklet i can send you if you want to give me your email. it has a little bit about everything. from how to choose your rats, how to choose your vet, common issues with raising rats all the way to making your own toys and treats. people have told me it was very helpful when they were getting their baby rats from me. its due for an update but i haven't written one yet and this is the best i cna offer for now.
