Chickens as pets - best breed Sussex? Keeping smell minimum etc any advise?

Chucks will easily hop up on a 3 foot wall. That said, some breeds are more sedentary than others, and less likely to hop up onto walls or gates. These include the big breeds such as orpingtons, light Sussex, brahmas, cochins, Jersey Giants... which are all gentle, sweet natured breeds that make nice pets. However, the flip side of the coin is that they're all inclined to go broody, which means they'll disappear off for weeks into a nest box, will likely need to be coerced out to eat and drink, and there'll be no eggs from the broody hen for weeks.
The bog standard red rescue hens are often good with people, friendly, great layers. But... their lives are shorter, and they are athletic... well able to hop a wall, and very likely to too!
As for getting chicks that don't have a mother hen to mind them, they can't go outside until they've feathered up... 3-4 weeks of age minimum, and that's in the summer. I would not be leaving anything other than fully feathered older pullets outside at this time of year.
Another major disadvantage of getting young chicks is that for most breeds, and certainly for the mild-mannered breeds mentioned above, you will not know if you're getting males or females.
My advice to you as a beginner would be to get point of lay pullets that have been hatched in an incubator and reared in a brooder with plenty of human interaction.
Be careful who you buy from... we have learned to only go to people who have a free-range smallholding, where they specialise in breeding only a small number of breeds, of good quality, and which have a good bit of human interaction... and who are only interested in sending you home with what you want, rather than chancing it that you'll end up with a cockerel or two!
I'm assuming you're going to have a shed for them to roost in at night, and which will keep them safe from predators. Also that you have secure and predator-proof area for them to root around during the day. There will be a smell no matter how clean you keep the shed, because chickens smell. But, the cleaner you keep the shed, the less pungent the smell. You will also need to have a parasite control procedure in place... worms internally, lice and mites externally.
Automatic doors are great, but not cheap. We have our automatic hen door on a timer, not opting for the light-triggered unit for various reasons. But... you categorically cannot go on hols and leave the chucks without daily checks, even with an automatic door. Water needs to be topped up, food checked, wellbeing of hens checked, eggs collected, shed cleaned.
The automatic door on our shed decided not to open a few weeks ago, on the one morning I had gone out to work pre-dawn. The hens were stuck inside until I got home. The only thing worse than a disgruntled hen, is multiple disgruntled hens. You have been warned