My suggestion will blow your eyes out of their sockets. Think of it like this, it's better to invest in a good ergonomic setup than have a terrible back at an older age. Your surgeries will cost 10 times more than a good office chair.
You should be on lookout for secondhand Herman Miller chairs once you have the necessary funds. For now, just buy a chair that fits you. Go and try different ones within your budget. Sadly, with that amount of money, your chair will be unusable after about a year and you'll have to buy a new one.