1) I'm only allowed 6 hens in our neighborhood and currently have 5. Is it easy to resell (or give away) any roosters or excess hens that are 2 years old - and not good layers...
Yes if they are free or very cheap

2) Is it ok to get the eggs via post or am I less likely to get success that way?
Its ok but expect only say 50% to be ok

3) Is it true I don't need an incubator as my broody would do all the work in raising them?
Yes true

4) Is it ok to mix breeds?
Yes

5) Is now a good time of year to do it?
Perfect time

6) I already have a big mess with half my backyard partitioned off for the chooks and they dig all day long - is it likely to get any worse than no grass left? (The soil level has to topped up yearly as it all seems to go??)
At least they air rate the soil

7) Is my broody less likely to go broody again if I put eggs under her? Will she be 'cured' and laying eggs soon after the chicks come?
She will go broody many times yet its built in

8- Should I get a dozen or is that too many?
Most only sell by the dozen and allow 50% to hatch

9) I only have 1 little coop - is this a mad idea (my hubbie just shakes his head in disgust!!!!!!!!)
As long as they have enough room its a good idea the more chooks the better