I love your "dream"!
Being a romantic at heart, I see this happening, when you put your mind to it.
First of all some facts to consider:
1. Bending you knee, running, entering a car in armor ist NOT (repeat NOT) possible. (I can sit in my armor, but it takes a considerable amount of athletics on my side and quite some time ...)
2. Finishing your suit in a week IS possible. You can't do anything else, of course (Like going to work or doing the household) ...
3. Most of your squad will gladly support you, but not everybody wears a TK. Keep in mind that getting together so many TKs in one place is a huge undertaking. The biggest group of TKs I ever had the privilege to be part of consisted of 9 Stormtroopers. (Normal troop, not a big event!)
4. Look for help in the early stages of planning. Find others who benefit from helping you, like sponsors looking for publicity(local car dealer?), a local TV-Station, a newspaper. They can pave the way for you. Everybody else WILL bow out sooner or later! Don't rely only on friends and aquaintances. It's not them who will propose, so they will lose interest when it becomes hard work and time consuming.
5. Do this stunt on a weekend, preferably on a sunday. More people will have some time to spare at the weekend in contrast to the middle of the week, when everybody is working. Okay, your girlfriend won't be at work, too, on a weekend, so she can't be lured outside during her break, but you will find another reason ...
6. If you need to spend a lot of time preparing this and you have to leave the house more often than usual, don't tell her silly lies. Tell her that you try to do something big and that she will just to have to wait. You don't need to tell her details, but make clear, that this is something you do for her or for both of you. Keeping something like this a total secret, will only make her suspicious and will influence your relationship!
7. You will need mics and amplifiers. Otherwise nobody is going to hear anything of the dialogue, your proposal or her answer.
8. If you are thinking along the lines of a camera team preparing a setup for a commercial to hide the preparation for your proposal, why not make it into a car commercial for the Ford Escape? The car will be in plain sight the whole time and really nearby. I hope you have connections to a local Video Production Company, otherwise you won't be able to pay for this.
9. Police. Tell them. Invite them. Talk them into participating. Masked people wielding Weapons in a public space in a crowded area will lead to problems! Try to let the spaectators in on the joke very early. Hang up huge posters an hour before the thing happens. Place some uncostumed friends of yours in the crowd and tell them to spread the word, keep everybody informed about what is going to happen, except your fiance. Don't rely on the good will of the bystanders!
This is doable, but not on your own. You will need a small army of helpers.
If anything comes out of your "dream", please keep us informed. Like I said, I'm a romantic at heart.