Two tips and a piece of information:
1. Air flow air flow air flow. There’s nothing more important to cattle than good ventilation. Calves need to be dry - sheltered from rain and with fresh dry bedding to lie on - but a cool breeze will harm them far less than a humid, stagnant environment. The ideal is air flow above head height, which draws the air constantly. So something like a gappy ridge construction, or two open flaps high up at different corners, making a flow across the calves’ area. But an open-fronted area, provided they can always retreat from any rain blowing in, and there is dry bedding there for them, is better than a sheltered inner corner with no air flow. They don’t actually need to be warm per se, as long as they are dry, have plenty of fresh dry straw, and are fed warm milk.
2. Don’t buy calves less than a month old. Generally the ones that are going to die start to be sickly in the first month. If it looks healthy at a month, there’s a good chance it will live. I lived with a third generation cattle farmer. He was in his sixties and had been buying in and rearing calves (alongside his home bred sucklers) since he was 10 years old. He never bought calves at less than a month old, unless they were going onto a freshly-calved cow. And I mean never. I worked with him for nine years and we had just one hand-reared calf die in that time, out of between three and six bought in a year. (Admittedly, once my Jersey was established, we reared them on her, which is even better of course. But he rarely lost them before I came along, and said the two biggest factors were the two tips I’m passing on to you here.)
3. Bottle / teat is better than head down sucking from an open bucket. The farmer I’ve mentioned did use a bucket, but he knew what he was doing and which calves to buy. It just avoids a lot of problems to have them drinking through a teat - although of course you do need to be scrupulous about keeping the teats clean.