No matter when you choose to propose, there will always be issues/concerns with money, timing, careers, etc. There is no perfect time.
In regards to your ring question:
1. I concur, you should have purchased the ring before filing the FAFSA. If possible, do it now and amend your SAR. Any other major purchases? Do it now and amend, amend, amend. Then again, why am I giving you financial advice? You have a crapload more than me in the bank. I am so jealous ;o)
2. Consider pricing your 15K - 20K diamond on Bluenile, which typically has much cheaper prices. Then take the price estimate to a place like Shane Co., which will price-match Bluenile, and voila, you've saved yourself several grand. Furthermore, if Shane Co. (or insert other company name) messes up and can't match your diamond exactly, they'll head over to that lovely diamond city in Europe and you'll find yourself with the upgrade closest to your original request. Voila, another grand or so.
3. Don't leave the paperwork where she might find it prior to proposal. She'll figure it out, but she won't tell you since she doesn't want to hurt your feelings. And then she'll mention it to would-be proposers on anonymous chat boards to help them avoid making the same mistake her husband did ;o).
Best of luck. Marriage is harder than it looks and much more exciting, too.