Well, I would not buy something you cannot afford outright. I would charge it and pay it off in one month in one payment, so not only do you get points for the purchase, you have protection on your purchase. DON'T PAY CASH if you can avoid it. Wire transfer is ok in a few instances, because you can still get some prtection, for a few decent vendors who will still refund you. But credit card (not opening new) is the best and safest because then you have recourse.
There's nothing wrong with buying something small, or even just a plain or diamond band now, and when you guys are more established later getting a traditional eng ring then. Just don't go into debt.
What do you envision and want, and maybe we can help you find something close to working in his budget? If you get a pave setting you will eat up a lot of your diamond budget, so I would say to focus the money on the best diamond you can get, and get the plainest, simplest setting you can now, and closer to your wedding or even an anniversary, get a set that you love and reset the stone into that. That way you can focus the budget there now, and save for a setting you love.