Hi and welcome!
Firstly, if only everyone would put so much effort into making the right choice about getting a puppy, the breed, lifestyle considerations etc as you are doing. The very fact that you are considering all these aspects BEFORE you purchase a puppy tells me you are going to be a good owner
There never is going to be a 'perfect' time to get a puppy/dog for most people. While we all would love to be in the position of spending every moment with our shelties, sadly life gets in the way and mostly we have to earn a living, look after families etc. It is perfectly possible to have a well balanced dog without having to spend 22 out of 24 hours together. As regards changing homes/lifestyles, it would not be ideal with a puppy but the older ones seem to adapt usually very well to a change in circumstances. Mine are left with their stuffed frozen Kongs and the radio if I'm out for more than 4 hours and they almost push me out of the door so they can get started on them! An abrupt change from busy household to being left for longer periods is not desirable but given the chance to get used to being left and a new routine, most shelties adapt very well. Generally, the more confident a dog is, the better he will cope with changes. In my experience shelties usually housetrain quite quickly.
I would suggest that it is much better to start looking well in advance. Even if you are not interested in showing, visiting shows is a fantastic way of looking around to see which dogs (and breeders) appeal most to you. Sitting with a catalogue by the ring watching the dogs being judged will give you an idea of what they are really like and you may well find yourself preferring certain dogs over others which will give you a starting point on who to start approaching with puppy enquiries.
Crufts is coming up in Birmingham in March but closer to home the following week is the Shetland Sheepdog Club of Wales Championship show which usually has a couple of hundered shelties there to look at.
http://www.sscw.org.uk/4.html
If you visit a show and find a breeder whose dogs appeal to you, when they are not busy getting a dog ready, you can go up and tell them how much you like their dogs. Even if you don't get a chance at a show you can call or email later on (usually all the adresses and tel numbers are printed in the catalogue). It really does help to see the dogs in the 'fur' so to speak as a photo only tells so much. It is wise to start looking early and good breeders will much prefer someone who is planning in advance and not buying on impulse. One tip is to beware of the breeder who seems very keen to sell you a puppy without asking very many questions as responsible breeders put too much love into their puppies not to make sure they are going to good homes
When talking to breeders be honest about what you want from the puppy. i.e to train for performance rather than showing etc. Explain you are not ready yet but are making enquiries and give your time frame. Sometimes they will have a waiting list for puppies so you might have to wait anyway but they will probably have a reasonable idea of their breeding plans for the coming year so can advise you if they are not likely to have what you are looking for. I would phrase the question of why they bred the parents as asking what the mother is like (in personality etc) and what did they particuarly like about the sire. You will probably find all this comes out quite naturally in conversation.

There are not many sheltie breeders who churn out masses of puppies mercifully(!) but again, long term research certainly helps. This will also probably come up in conversation without needing to ask directly. Don't worry about asking how much - it's a reasonable question as you need to know!
In the UK, the main issues are CEA and hip dysplasia along with PRA. CEA is non progressive. Breeders will have their litter screened at 6-7 weeks. An affected puppy will not get any worse and unless the problem is very severe, it does not affect quality of life. PRA is thankfully pretty rare so ought not to be an issue but it's certainly worth doing as Sophie has suggested and looking at the KC website for health test results. If you want to do dancing/agility etc it would be prudent to purchase from someone who is hip testing as sadly hip dysplasia does affect some shelties. The testing does not give a guarentee as the condition can arise even when the parents have low scores but it certainly helps avoid it. Currently (as of nov 2010) the average score (from 460 shelties screened) is 13 and breeders should aim to breed only from animal below this.
This gives a brief overview and some useful links:
http://www.essc.org.uk/health.htm
Sorry about the length of this post

but I would say shelties are loving, intelligent, tough dogs who will relax with you and do not require as much brushing as would appear (I do mine once a week). They are gererally good with people, kids and other animals and adapt to most circumstances. They are small enough to be picked up easily but big enough to be a 'proper dog'. On the downside, they shed and they bark, especially when excited and they like to be with you at all times (underfoot). And they are addictive
Wishing you the very best of luck with your quest.