I am one of those single adopters who have never made it back to work but I also would say don't wait 7 years. Between child benefit, child tax credit, DLA and Carers' allowance we muddle through. It's not what I imagined myself doing, but it works for us. I probably could have gone back to part time work by now but my children need the stability of me around more than we need money.
Try and get as much paid off your mortgage as you can over the next 2 years maybe - but then apply.
Firstly, the process can take a while; the agency may say six months, but that is not that common and there are many variables which can slow a process down. Plus, that six months is to approval panel. For many adopters the longer part of the process is waiting for matching - and even when you are linked to a child it may still take months.
I held off for a couple of years to get myself into a better financial position - which was worthwhile - but I also used that time to try and plan ahead to avoid future costs. I re decorated, had a new boiler installed, changed my car... I even stockpiled many of the more expensive items from my weekly shop by buying one extra each time. We lived for a very long time without needing to buy toilet rolls, shampoo, washing powder, cleaning stuff etc.
I also think that if you have a strong support system now then use it sooner rather than later; you don't know what each of their situations will be in 5 or 7 years time - they may be unable to provide as much support for you by then.
I think planning ahead and thinking clearly about finances is very important, but if you do take some time before taking the plunge then use that time to prepare yourself in other ways too - by learning about trauma, mental health, our education system (especially the SEN system), attachment, contact ... and gradually educate those around you too!