For items I think that tanky stats are probably the best, utility items are also necessary for Thresh, good items are like Solari and Mikaels which both give tanky stats along with utility.

For runes I go mixed tank to protect against all types of damage. Personally I typically run against heavy ad or ap match ups quite a lot so I go switch my hp seals for armor and MR a lot.

I find Thresh to be good with Kalista, and actually most adcs. I find Kalista especially good because of Kalista's ult, he gets thrown and he can lantern her, great for an all in or an escape.

When teamfighting as Thresh don't go for the engage, but instead follow up the engage by using the second active of Q to get right in the fight and tank UNLESS in the laning phase you stomp and get fed with a lot of gold to be your teams main tank. Also, in a fight don't be scared to die in order for someone else to escape. I don't care your KDA just yell #Wurfthesacrificeisreal in all chat