There are as many different answers to this as folk who keep pigs

You'd usually be aiming for around 80Kg liveweight for going to slaughter, how quick your pigs reach this depends on breed, feeding, weather etc.
Normally, a traditional breed will get there
around 6 months (assuming 1lb of feed per month of age), if it's really cold or they're being fed less it might take a bit longer, commercial breeds will be quicker growing.
Putting them in with Kunekunes........ I don't mix mine. Mainly because the big pigs root the grass which the Kunekunes need for grazing and their feeding needs are different - but I'm sure some folk do mix them. Just be aware of entire boars running with gilts - Kunekunes especially become sexually mature quite early!
Weaners cost anything from £2 at some markets

to over £100 depending on what breed and where you buy them. My recommendation is to find a good local breeder and buy there

You'll probably pay £50-£60 for a good weaner, but it's worth paying a little bit more for good stock IMHO. Have a trawl back through the pig section for lots of discussion on this

HTH
Karen