Some advice from experience:
You'll want very stable water conditions. Always understock if possible and introduce the fish into a completely cycled tank.
Live food on occasion really pops their colors and keeps them strong. Try to stick to insect-type foods to avoid parasites that can ride in on tubifex and blackworm cultures.
Watch for camellanus worms. They are common in angels for some reason.
The first few days are the hardest for a newly moved angelfish. Lights, movement and other stressors can take their toll on the immune system and make them more vulnerable. Once settled in, they are much hardier.
Black, especially Double black angels, are very fragile and may die if you look at them sideways.
The hardiest strain is said to be the marbled. Silver and Zebra are next. Zebra is my favorite.
You'll want both biological and mechanical filtration in good amounts as you would for any fish.
Keep the nitrates as low as you can.
PH isn't the problem it used to be. Captive-bred angels are usually dozens and dozens of generations away from their wild ancestors and have adapted well to dechlorinated tapwater conditions.
Any trace of ammonia or nitrites will be evidenced by fin clamping and heavy breathing. Unlike many hardy aquarium species, angels may not make it through a mini-cycle.
Provide territories and or tall caves. Angels can get very combative and dividing up the line of sight in a tank is a big help for this problem.
My Zebra.