I would hold off man. Get it together, find a job etc. Your condition is that bad that you literally cannot work? Sciatica is legit, but still man.. 2 years w/out a job and you mentioned that you like to get loaded, go to raves, hang out etc. People are right, the diamond thing is a racket but that said, if you buy your girl a pinner little rock it'll be a bummer. There are ways to get a good rock and setting that meet the high end of the C's w/out breaking the bank. Source out your local jewelry district, wholesale jewelry part of town and go haggle with some Arabs all afternoon. Let her pick it out. Don't be a schmuck and think you know what she likes because she doesn't. Set a budget and go shopping together. If you go to Zale's or some place like that you are buying an already horribly inflated product at a massive markup. Shop smart and you'll get a good stone and setting that she loves with all the papers and certifications, you insure it via renter's insurance for the value and call it a day. Don't go to crazy on a wedding either as that is an even more massive waste of money.
But for now, I'd get back to work, save some money and hold off. Being engaged for a long time is just weird. Get engaged, set a date, get it done and move on.