mummytoo welcome.
One of my kids was born via IUI and the other is adopted. I wish you all the very best in your journey. 
mummytoo I agree that sometimes social workers do send mixed messages about value of parenting experience. For us the fact we had a birth dd and had experience of parenting was definitely something that went 'in our favour' when we were chosen for our son.
I am a bit confused about the age of child you would like to adopt and when you said We expected to be given an older child if successful, and we called to ask what age gap to the little one would need to be before even applying...we were thinking maybe 3 or 4 years minimum.... they suggested applying at the end of this year.
Do you mean you expected to wait and they said end of year? Or did you mean older than your current children? You need to keep in normal chronological order. So birth children do not 'lose their place'. That is if you have a younger child of 2 and new child would need to be younger (by at least 2 years) than your youngest child. If you applied at the end of this year and by then your younger child was three and a bit you could adopt a baby or perhaps a toddler. Quite a lot of people also want to adopt babies and toddlers but also, I know a lot of people who have adopted older children too, so of course people are needed to adopt all age ranges.
I would echo any advice here that says to find out all the facts about adoption and if this is best for you, as a family.
I would also soul search quite a lot about whether you are ready not to have another bio child (bio for you or your partner) and make sure you are over that stage of life. It's not just about DNA etc is also whether a child has had a tough start to life, perhaps exposure to drugs or alcohol (or both) etc etc.
I did a whole lot of soul searching I ended up having thousands of pounds worth of failed treatment in a bid to have number two, but then I had fertility issues and your need of treatment is not for the same reason so may be easier for you.
Finally, I was done with treatment and ready to adopt and it has been brilliant, really I could not imagine it having gone better! However, I think I lucky, our son has adjusted very well. The hardest part for me has been the reaction of my birth dd who has felt very jealous and has found it hard.
If you decide on adoption I would definitely consider investing in an extension or converting a room, maybe divide the biggest bedroom or whatever you can do. A simple room conversion may be reversible. My friend did this for her kids. They had bunk bed type things with storage under the beds and very compact and bijou but good for little kids if old enough for top bunk! When the kids left home she had the room divider removed.
If adoption ends up not being your chosen option you could have a room you could use for a child born of assisted conception.
Adopting then sorting the house out sounds fine but what if you do end up with no money to do it. Will your current kids end up resenting the fact your new child gets their own room and your current kids share? They might. My birth dd is very jealous of her new sibling and although she loves him it would be hard for him to be perceived to have a lot more than her. As the first born she has the bigger bedroom!
Also my kids had to share at their grandparents this week and our 4 year old ds was still awake when my hubby went to bed at about 11! Sharing is something we can do on hols and family weekends away but long term would not work for my kids. Your kids may be different.
Just some things to think about.
All the best.
PS I am nearly 50, and we adopted last year (2014) so I am wondering roughly how old you non-spring chickens are!