Ref appliances. I always buy Bosch or Neff. I usually get something midrange, not the cheapest and not the most expensive with too many controls you won’t use. Ie oven, dishwasher, washing machine. See how long the warranty is. It’s usually 2 years on better brands. I have bought direct from AO and John Lewis.
Samsung are very good for fridge freezers. My friend recently got a Samsung washer dryer as it had a 5 year warranty. No problems yet.
You could take out a subscription to Which, they give to lowdown on appliances and kitchens. Last time I looked they rated Ikea quite highly.
I have used Crown Imperial in the past. They are priced very well and reasonable quality. My friend has Howdens, good value if you can buy it direct at trade price (trade price is usually 70% off the retail price), so you can see why kitchen fitters like to sell you a Howdens kitchen.
Our last kitchen we used a company that made their own cabinets. I was able to choose my own cupboard interior (usual choice is usually white, silver or oak). I also asked them not to put in all the predrilled holes for shelves, or told them where I wanted the shelf holes to be.
Check out the thickness of the mdf for the carcasses and the backboards. The thicker the better.
Kitchens are priced so that the 600 mm base cupboard is a very reasonable cost. When choosing the range they will probably use the 600 unit to compare. But if you need anything not a 600 unit the cost per size is usually way more.
So if your design can use mostly 600 base units, you will keep costs down. Don’t get sucked into buying those carousel units and internal units from the kitchen retailer. You can buy them much more cheaply on the internet.
Last thing, inspite of all I have said, in my last kitchen I did buy deep pan drawers (not cheap) but I bloody love them. I’ve been involved in buying about 5 kitchen over the last 15 years, not a professional though.