Just my opinion but I'd say keep the car. Correct me if I'm wrong but I think at the moment with the price of early commodores you'd be pushing to get 4k. Your hearts in the right place though and if you need to sell it then you need to sell it. But work it out together.
When I was 18 I was "Lucky" enough to to get rear ended in my LX torana I got $1300 to fix the car. Used $250 to fix the car $600 for an engagement ring and $300 for a lawn mower. Luckily she said yes.That was 12 years ago and the Torana and lawnmower are long gone
Our wedding too only cost about $2000-$3000 And neither of us wish we spent more, theres so much more in life that needs the money put into it like a house deposit.
Hopefully she doesn't read this but first I'd try and scrape up enough for a nice ring in the style she likes...Shes probably shown you already but if not next time you walk past a jewellers have a browse together and she'll point out the ones she likes. I wouldn't go crazy and get a 3-4k ring, probably wouldn't spend over $1200. And then pop the question!!
Up to you how you do that I did it so badly I'm lucky she said yes. Plus you have to do that stuff your way because its gotta be from your heart
Anyway good luck to you both! Hopefully we see a build thread here soon with her calais...you'll need it for the bridesmaids