we chose the site first, that is the most important bit (sorry, don't want to say where). We just googled with a list of requirements (eg, location, coast/country, swimming pool, playground etc). It depends who has pitches available. There seem to be a few available on our site, I think some people are selling up because of the crunch
it had to be within 1.5 hrs drive from home, too far and you get to go there much less. It really was location, location, location for us . Then with a list of possibles, we drove round them and picked the one that felt the best/ticked the most boxes
the actual caravan is much easier after that, and not the biggest decision IMO
you can buy one from the site itself and pay them a fee to place it and hook it up etc (simplest option, they make it very easy for you, but probably the dearest option). Most sites have a selection of new and used vans and will add on the fee to the price
or buy from outside and pay to have it transported
like buying a house really !!