From my experience, show-type cockers are happy to go as far as you'd like on a walk. I'm by no means super active, but mine have never decided they've had enough of a walk, sat down and refused to move. 30 minutes to an hour offlead over at the park is perfect for my two, although they never complain if it's a bit longer or shorter than usual! They also aren't fussed if they miss a walk, I know with some breeds you definitely can't get away with missing that one day, as they will literally be bouncing off the walls otherwise. Ours are chilled out in the house, active and curious outdoors.
It's hard not to completely fall in love with cocker spaniels! My dad (who is a fairly big guy) wasn't overly keen when we first suggested a spaniel, he actually wasn't a dog person at all before. Now he's totally besotted with them. They are definitely not delicate little frou-frou dogs lol!
I also work from home, they can be prone to separation anxiety because they just love being around people. Of course, as long as you build up leaving them alone they should be absolutely fine to be left. But they are very sociable with people and other dogs, and really wouldn't suit a life where they are left on their own the majority of the day.
Cockers are very trainable, and whilst they would not excel quite as much as their working-line counterparts at things like gundog training or agility, they tend to be very food orientated and will work for treats. I've found clicker training to be a useful tool for mine. It's important with all dogs to teach them from an early age that jumping up at people will give them no attention. Recall is also very important - thankfully the show types tend to be less drivey than the workers and Springers, and in theory should be less likely to run off after birds and squirrels. They are still spaniels after all though!
Regarding breeders, I'd have a look on Champdogs and maybe put yourself on some waiting lists. A good breeder should be happy to answer any questions you might have, they should be passionate about their chosen breed and will be asking you questions too! Always a plus if the breeder is actively showing their dogs (whether that's in conformation, obedience, Good Citizen Dog scheme..) to prove that they are breeding quality. Show Cockers should be health tested for Progressive Retinal Atrophy and Familial Nethropathy as the bare essentials, eye examinations and hip scores are also great to have done. Definitely do check the inbreeding COIs with the Kennel Club's mate select as we are seeing more and more issues with very closely inbred dogs of all breeds. It shouldn't be too much of an issue with cocker spaniels though as they are very populous and have a large gene pool.
Cockers come in a huge variety of accepted colours. There is a lot of controversy around breeding for the 'sable' colouration - it's not an recognised colour in the breed, and as you can imagine there are a lot of unreputable breeders trying to cash in on this 'rare' colour, and many often charge much much more for a sable cocker simply because it is unusual and sought after. I would avoid breeders that are purposely breeding for colours that are not recognised by the breed club. Health and temperament suffers when a breeder is purely focused on creating pretty coloured dogs.
I will also add that their lovely, silky coat does need regular brushing and trips to a professional groomer to keep them looking nice and matt-free. But I think this is true for any spaniel that isn't clipped super short. Their coat attracts all sorts of seeds and burrs and you need to be especially diligent with checking their skin for grass seeds, all that long hair can easily hide it! On the plus side though, a well maintained coat doesn't shed an awful lot (they're definitely not dropping hairs like pine needles!) and they are wonderfully soft to stroke. Those long ears are a blessing and a curse - as you can imagine they drop them into everything - in the grass, the mud, in their water/food bowl! Ours wear doggy snoods when they have their dinner so their ears don't get in their food! They can also be prone to ear infections due to the long ears, but if you can keep the ears clean and dry, you should be able to avoid this. Thankfully ours have never really had ear issues.
If you have any more questions I'm more than happy to go on and on about cocker spaniels! The breed has absolutely stolen my heart.