It would be a good idea to try the armed forces adoption agency when the time comes, www.ssafa.org.uk/how-we-help/adoption you're guarunteed a favourable reception on that front!
Yes your children need to be a certain age - the standard age gap rule is that your new adoptive child needs to be at least 2 years younger than your youngest child. However some agencies wil have a slightly different rule, so you can check that with your potential adoption agencies. Most agencies are therefore not going to let you begin the process until your youngest is at least about 3 and a half to 4 years old, so you can be approved for a 0-2 year old child with a 2 year age gap. I would suggest at least a 3 year gap as a good thing, just my personal opinion
Birth parent involvement - in terms of contact, you're usually talking one or two letters a year going back and forth. The birth parents may reply to your yearly letter/2 letters, or they may not. If you're comfortable with this level of contact, you'll be fine, becaus this is the level of contact the majority of children are placed with. A small number of children need actual visits (normally 1 or 2 a year), usually older children. You also need to consider siblings - how far are you comfortable keeping your new childs sibling relationships going? Would you be happy to do sibling visits?
Aside from contact, you have to talk to your child as they grow up about their adoption and why they were taken away from their birth parents. You have to support your child with whatever they are feeling about their situation and birth family
As your child reaches their teenage years and beyond, then you need to start thinking about more contact - in terms of, will the birth family seek out my child using social media, or will my child seek them out etc etc
How this impacts on a family totally depends on the situation. For an older child, dealing with their emotions surrounding losing their birth family can have an impact on them settling in for sure. For a small child, the main issue will be their grief and confusion at losing their foster family, not their birth family. Ongoing letters may have a big impact or a small impact, depending upon the childs feelings, how it's managed, what the birth parents write, etc. It can't be predicted I'm afraid so up until you actually get matched with a child, you can only make decisions on what level of contact you are comfortable with and how you are planning to talk about it as your child grows up.
Hope that helps