Hey OP
A few bits of advice for you, hope it helps and I don't waffle too much... It’s more practical advice for the setup than anything. For buying the snake it depends where you are really. Crystal Palace Reptiles/Snakes and Adders/Coast to Coast Exotics/ Evolution Reptiles and loads more (that I can’t remember) are well known.
Have a look online for good breeders/rescues. Shops will always be much pricier.
Reptile Forums UK has always been a gold mine for me for advice and classifieds. In terms of posts it’s much less active now but you can still search the old threads for advice (once you figure out the annoying search function). There are plenty of detailed care sheets on there on the snake section for loads of species.
Royal pythons are famously very tame but can have feeding issues. The key is to monitor their weight regularly. Most start feeding again and it's rare for them to lose any serious weight even after several months. Saying that, most are good feeders.
Corn snakes have great temperaments too and will often feed more reliably than royals. They are lovely and active snakes.
King snakes tend to feed very well and are handleable but can be feisty, especially when young. They are nicknamed the dustbins of the snake world for their love of food.
As a good general rule female snakes are larger than males, in both girth and length 
Shopping wise:
Swell reptiles are probably the best and cheapest online shop for equipment.
What you would need to buy for a ‘standard’ set up:
Vivarium - most brands are similar in design and quality. For adult corns/kings/royals a 4ft x 2ft x 2ft is the usual recommended size but the bigger the better!
Heater- this can be confusing at first. Basically you need a heater and a thermostat to control the heater. There are three standard options:
1. Heat mat and mat thermostat - cheapest option and works well. Heat mat should be as long as around 30-40% of the length of vivarium. Easiest option for newbie owners!
2. Ceramic heat bulb with a pulse proportional thermostat. Ceramic bulbs don't give out light but give out a better all round heat. They are normally the most expensive option.
3. Basking heat bulb with dimming thermostat. Basking bulbs will provide a basking spot but also light up the enclosure. Middle cheapy option.
The thermostat is a really important piece of kit to buy, and is left out of some starter kits because the price puts people off. The thermostat controls the heater and prevents it from overheating the enclosure. Without the thermostat the heater can get too hot and could burn the snake or potentially start setting things on fire.
Heat bulb guard - if using a ceramic or basking heat bulb (covers the bulb with a metal cage and screws into the ceiling of the vivarium)- some snakes will wrap themselves around the bulb and burn. Exotic vet bills are horrendous 
Digital thermometers x 2 (one at hot end of vivarium and one at cold end) I get mine from amazon- much cheaper then reptile shops. Get a digital hygrometer too. Just check the reviews. Don’t buy the stick on or round analogue dial thermometers/hygrometers- they are shite.
Substrate- aspen is commonly used, cheap and easy to clean. Make it deep enough in the viv for the snake to burrow.
Couple of hides - can be reptile hides, tubs, things to hide under/sleep in. At least one on cold end and one on hot end.
Fake leaves and logs/decor - lots so snake can hide. Also a nice big water bowl.
Don’t buy heat rocks - really bad
Hope it helps, sorry to ramble. Can’t sleep, drank too much wine!