I’m the world’s biggest cheapskate so I would recommend that you don’t buy anything new unless there’s a really good reason for it.
Harnesses. Know what you are buying. Go check out some harnesses in other people’s cars and figure out what you like. Pull up/down, spring loaded adjusters, width of shoulder straps and latch mechanism all need to be considered. Is harder to get these used because they have expiration dates.
All clothing can be bought used. Shoe sizing can run a little screwy tho. If you are looking at ebay auctions, have them measure shoes, suits or whatever. Don’t just go on the manufacturer’s size info.
Expensive suits use flame retardent material. Cheap suits use flame retardent “treated” cloth. Cheap suits lose their flame retardent qualities after multiple washings. So nice suits retain their flame resistant quality even if they are used and a cheap suit might not.
Seats can be bought used but know what you are buying. They too expire and come in lots of different shapes and sizes. Go sit in lots of seats to determine what you like. Then when you see seats for auction/sale on the Internet check out the seats on manufacturer sites carefully.
+1 for always buying from sponsors when you can. Even if it costs a couple bucks more, supporting sponsors is important. Most of them are at the track each weekend rescuing one of us from some unexpected “Christ, I broke my upper widget. Where am I going to find an upper widgit on Sunday morning?” crisis. You’ll be the one in need of rescuing soon enough.