We did an IKEA kitchen 2 years ago.
We had someone we know fit it with DH so I couldn't tell you about fitting costs or standard - but from what I hear it's very variable, so if you know anyone who could do it yourself it might be simpler to go with them.
I spent a LOT of time online looking at kitchens and we also went round the room cupboard by cupboard deciding what we needed storage for and where it would all go.
I also posted my design on here and got lots of useful feedback.
I did pay a kitchen designer to draw me up some plans, but after 11 plans, gave up with them, because they couldn't come up with what I wanted. We did the plans ourselves and are very happy with them - there's nothing I'd change except perhaps a bit more under-cupboard lighting.
You can check the stock online at IKEA before you go, I was incredibly lucky and went with a friend on a weekday in July and everything was in stock, we did 2 trips to the car (we have a big car!) and loaded it all up and brought it home that day.
I did have to go back to exchange a couple of doors and to buy some more interior fittings, but didn't do TOO many trips as we are an hour away from our nearest IKEA.
I think you have to visit to order, we went in once with the DH to talk our initial plans through with their designers (go early so you don't have to queue) and to choose which style we wanted.
Then I finalised the plans online and went back, and you go up to counter with your plan, go over it one last time (they made really useful suggestions about number of knobs/handles etc.) with the staff and then they put it all in the system.
Some of the things you have to go round and collect yourself (this takes ages) and others you go to a special counter to collect.
If you do your design online you can already see the list of all the components you need and check the stock yourself.
In the end, if you have to go back for a door, it won't hold up the fitting process (but a whole cupboard would iyswim).