Look carefully at areas that show wear.
Drypants: Look at the knees, ass and seals
Cags: Elbows, wrist and neck seals, under the arms. Check inside to see if it is coming apart
Helmets: Do not buy second hand, there a good quality cheap helmets but this is something that should only ever have one owner.
B/Aid: As buoyancy ages they loose buoyancy. If it is older than three years it can have lost half it's buoyancy. Test it. In most cases this won't matter but it something to keep in mind that B/aids have a life span.
Blades: Lenght, feather and size of blade face. Look for obvious scratches or cracks or any sign of glueing. Do not buy the biggest blade face you can. Try out other people blades first if you feel pain or strains there is a good chance that the blade face is just too big for you.
Boats: Only used on grade 3 and surfing is the boating equvialent of elderly lady owner didn't drive much. Look for wear on the hull, kettling (Creek boats especially kettle(bending in) with age around the seat area), signs of repair to the out fitting, plastic welds and of course try it out if you can.
Order to buy stuff: Cag, booties/footwear, long johns wetsuit(the dry pant/cag versus cag/wetsuit combo is for another day, I still have my wetsuit from when I started, I do not have my first pair of drypants), Neoprene deck, B/aid and finally a helmet. Now start looking for blades and finaly boat. The reason for this order is what will amke paddling more comfortable for you and what will benefit you more also if you decide to quit you haven't spent money on things you won't use at another water sport.