Start off with a couple of white pigs, raise them to pork weight and hire a hog roast and give it a try to make sure you enjoy all parts of the process before you commit to the full set up
It'll cost upwards of £5K for a basic set up, decent machine, gazebo, catering bits and bobs and a trailer to store/cart it all around in.....you could easily spend double though.
Breeding pigs to supply your enterprise year round will depend on how many roasts you plan on doing every week - 1 a week means approx 50 porkers a year and having them at the right weight at the right time (most machines have an upper weight limit - especially those with a spit) Assuming you can manage the feeding/stagger litters you'll be looking at 3 breeding sows each having 2 litters per year - a quick guess would see feed costs around £10,000 p.a......maybe more (for sows and raising weaners)
Once we factor in all our costs, we make around £150 profit from each hog roast......maybe I'm not charging enough
HTH