My husband and I married young. We bought into DVC at around your ages when we were married.
From a legal stance I would wait until you are married to buy DVC.
From a financial perspective, do either of you have student loans or other loans? You mentioned that your fiance would be going back to school, are you getting loans for that or paying cash? Do you own a home or plan on buying a home in the near future? I would not buy DVC without knowing what your full big financial obligations will be in the future, and looking at the grand picture of what kind of situation you will be in. Having a contract is a financial yearly burden, plus you have money tied up in it that you might end up wanting or needing.
I definitely would not finance DVC since the interest rate is crazy. Therefore, if you owe a lot in loans, or if you can't pay cash I would not buy DVC. DVC is only fun if you can hold onto it and afford it. It definitely isn't cheap. We have to pay dues each year, plus we do more trips and get annual passes now. Before we purchased DVC we did Disney on the cheap. We would stay off property at a cheap hotel and whatnot. Moderate Disney resorts can be quite cheap at times too.
Because of the time that we bought our resale contracts we did quite good "financially". To the point, that if we sold our resale contracts now I believe we wouldn't be out anything when compared to what we bought them for. In this current DVC era though, it looks like the most benefit comes with owning the contracts over a longer period of time. It would not be good to buy direct (or possibly resale) and have to sell it in a few years. I would make sure you are financially stable, so that you can hold onto your contracts for as long as you are interested in Disney.
The specific DVC route we went was buying resale. The only direct contract we own is for 25 points, so we could get full member privileges. Disney has since increased the minimum on direct points for full benefits to 100. I would recommend resale over direct.
Edit to note:
Just saw your update of buying. Congrats! There is a 10 days period where you can rescind your purchase if you and your fiance want. So, maybe keep thinking about all of it now that you have bought in. It's a nice window of time to make sure you both made the right decision.