Shit, this place is full of Casa Novas...
SS;
A friendly piece of advice. The fact that you will marry her after 5 years of being in a relationship, it's a huge event for her.
Women are way more emotional than men and they do appreciate the uniqueness of a moment. This is what I would do: I would celebrate the very moment in such a way, that she will forever thank you for making it happen for her.
How?
Old school is always the best way to go. It shows class and character. If you're gonna be down in Mexico, make it a date. Buy her a beautiful dress that makes her feel the most beautiful woman of the place, then tell her you wil meet her there. Women love anticipation.
Make sure that the place is has a dim light, candles are perfect. Make her wait 10 minutes and then approach her. Sit down and order the most expensive bottle of wine ion the house (make sure to order it like that: the most expensive bottle of wine in the house). Once you have eaten dinner, tell her you love her and get down on ONE knee and propose. When she says yes, IF she says yes... have a Mariachi come in from nowhere, so she gets thrilled. Then stand up and aloud, tell the world you're marrying the ONLY woman you love.
Thank me later for the savage session of nookie you'll get.
BTW, if you wanna repeat the nookie session, have everything shot by a DISCRETE 3rd party. Don't let her know she's being taped... and on the wedding nite, play the tape for her and give it to her as a gift.
That should do the trick.