Will eggs through the post be ok enough or should I find a local breeder? Eggs through the post will be fine, but I'd buy them from an online poultry dealer (there are lots) rather than ebay.
Should the run have a wire base to stop predators or will they not be able to scratch? You can use wire mesh on the bottom which is advisable if you have rats or a fox problem.
Do they need to be next to the other run so they can get used to them or away from the others? If you're going to run them together eventually, then yes it might help. Mine are sited all over the place but the different groups 'meet' whilst out free-ranging. My broody & her chicks would have been fine staying in the main henhouse but it really depends how they all react - you'll just have to judge it when you get there.
Will I be able to introduce them to the current group at some point,if so when? Probably! Possibly from when the chicks are a week old, it really depends how calm your hens are.
Do I need to sex them before they join the group, if so when will I be able to and how easy is it? No, you can run chicks of either sex with your laying hens. The cockerels may playfight amongst themselves but it won't be serious and the hens will keep them in line until they're at least 3 or 4 months old. You can start guessing the sexes from about 6 weeks but usually by 12 it's obvious.
And I like this book, it has quite a lot about breeding in it but you might want to get something a bit more detailed/specialised.
Thanks OldGregg, that's great. I've bought the book. I've never seen any rats but that doesn't mean they are not there. I've seen the occasional fox during the day. Do you think either of them would take a chick from the run during the day? The box has a closing door for night time.
It's unlikely a fox or rat would tunnel under the run during the day but you can never be 100% certain. Just ensure that the run is on level ground, no dips in the grass where a rat might squeeze under and check the edges of the run every morning to see if any holes have appeared - rat holes might only be a couple of inches wide. And if they're shut in at night they should be fairly safe. I have 6 chicks that have survived 6 weeks in a slightly rickety rabbit hutch and a homemade run without a base - and we've had a badger snooping around every night.