tothesea Car boot sales, NCT nearly new sales and eBay are good for preloved items. The smaller clothes (premature, newborn, 0-3 and 3-6 months) tend to be in really good condition. I guess that's because babies have so much clothing purchased for them in advance of their arrival that there's no time for wear and tear.
Our local NCT has a sale every 3 months, last summer I got my mate a second hand cot, a Moses basket and an assortment of neutral baby grows for less than £20, she was pretty chuffed and uses the cot at her MIL's when they stay over.
As to how much clothing, probably best to wait for sage advice from others on this page. I'm not a good example, as my poor DD had a very limited selection - about 8-10 all in one baby grows for each stage, with accompanying vests, a couple of hats (my parents and inlaws are pretty obsessed with having baby's head covered), a pair of mitts to prevent scratching and an outdoor all in one suit for days out in the pram.
Saying that I did have a reliance on the kindness of others for anything more exciting than that. Although I often found the more exciting items, fiddly with buttons in an odd place, but that's just me.