Rather than cardboard on those lower shelves, go to a family dollar, etc and buy some cheap peel and stick floor tiles, and put them on the shelves, so it looks nicer if someone opens the doors. The cardboard, appears to be a bit tacky. For your upper shelves, buy some contact paper, or some nice shelf liner to put your dishes and your food on. The outside is doable too. Take off the doors, and mark where they go Clean and sand them very well, fill in any dings, etc with wood filler. Resand that, so its nice and smooth. Same for the cabinet frames, and the drawers. Once they are all sanded and look decent, wipe them very well, and put them in a place where they wont get dirt blown on them, and prime with a sealing bonding type primer. Once dry, put on a couple of coats of cabinet type paint, in your choice of color. It will take a bit of time, so work on them little by little, only so many at a time. Follow the wait time between coats on the primer and also the cabinet paint can.

At least you have cabinets! Our first house, all we could afford!, had a wall-hung kitchen sink and an old gas stove. The sellers had removed all the cabinets 35 years before— because they had ants! Ironically, the ants walked through the house every spring on some kind of a migration trek!

Do a search for "kitchen cabinet pull out" and you'll find all kinds of pullout shelves and devices to maximize your base cabinets. Also look at spice shelves. I made some shallow shelves that fit spices and attached to inside of upper doors.

Look on this as a great learning experience. I learned so much from the bad kitchens I had in the past that, when I could afford my dream kitchen, I had a long list of things I wanted. “Making do” leads to your increased imaginative solutions. This can be fun! A gallon of paint would cover the insides of your cabinets, making them bright and clean and fresh, erasing all traces of the previous owners. Post some pictures and we can help with the rest of the kitchen.