I can only speak as someone who went through the process to become a foster parent, not one who actually took in a child. I have also spoken with some of the people from my course, who did take in children (who have since taken a break). But the process can be time consuming-classes, physicals, PAPER WORK-but definitely do-able.
The one thing that stuck out to me during my classes was that the instructors recommended that we only have children in our house that were younger than our child(ren). They suggested that since many of these children have been abused/neglected, that we needed to be cautious, as they could possibly project these behaviors onto our kid. Though you request children with no major "issues", sometimes the extent of their experiences is not fully known. My husband & I went into the process thinking we would take an older kid, but did not do the home visit, as I was pregnant during the class and concerned about the safety issues.
Once you become a certified foster parent they may ask you to take kids of any age because of need. But during the warm & fluffy process, it was recommended that we insure our children's safety first by not putting them into situations where they could be bullied, overpowered, and unnecessarily exposed to harmful behaviors.
Please be aware that social workers will call all hours of the night, the kids sometimes come with nothing, no cribs or formula for infants. You're left at 3am trying to buy them something, and the clothing stores where the dept. gives vouchers is not open. It is a financial commitment. Unless you neglect the children, you will most likely spend $ out of your pocket, that may never be reimbursed. For me, if you're in my house, you have to have what my child has, which most likely would not be reimbursed.
I don't want to discourage you, because I feel this is a wonderful calling, just be cautious. Right now it isn't for me, but I hope to be able to revisit foster parenting after I'm finished having kids. Good luck! Hope this has been helpful.