It's not so much that it's difficult, it's a bit time consuming as you have to have enough of the wood ( spare drawer fronts ) to make a cupboard door with. In fact you probably will need extra wood that matches as you need a frame for the door, if not for that but definitely for inside to deck out the new cupboard. Same applies to the smaller one. Thankfully it's raw pine, so it's very easily available. You will of course have a bit of extra wood from the hole that you cut for the sink, so the carpenter may be able to utilise that for the door! It also has to be finished with something waterproof by the way!

How about adding brass knobs? They have a traditional look which is lovely. What colour will you be painting the doors?

Aqua would look amazing with a white kitchen, but I think an off white would go beautifully with midnight blue. Something like F&B All White. I think Slaked Lime by Little Green would also go nicely too. My sister has a charcoal-navy kitchen and she chose the palest of greys and it does look good. The last picture shows Slaked Lime with navy.

There is no budget way to do the changes you would like. Adding a larger sink to the island would in all probability involve changing the unit below and the work surface if it is an undermount sink. I cannot tell from your photograph. These costs in labour alone would be considerable not to mention a new island worktop. You may be able to swap in your fridge/freezer by removing some of the tall units. Try and find out who supplied the kitchen if you like the style and colour and see if you can get a quote from them. They will also tell you what is possible.