Here are the perks of the Sig+ pass:
- 20% Merchandise Discount (valid at all Disney owned locations, except "Outdoor Vending" with a few restrictions on movies (sometimes), crystal palace and the like)
- 15% discount on food (Valid at Ghiradellis's and all Disney owned locations, except "Outdoor Vending", Clarabelle's, Smoozies (sp?), Gibson Girl, Tiki Juice Bar and Cozy Cones)
- Maxpass is included, which also means you get full resolution digital downloads of all photopass and on ride photos associated with your account and you can add as many photopass cards as you want. You also get the digital release form, so you can have them printed out wherever you want or use them to make t-shirts, etc.
- Most Downtown Disney locations will provide discounts for all passholders and the discounts will be the same regardless of level.
Since you will be parking at the hotel anyway, I would just leave the car there and not mess around with them park parking.
If you get your discount, you must be the one paying. The way to get around this is to have the other people give you cash or a gift card, then you use that to pay. Also, most CMs will make exception for a spouse (with the same last name) or a child, as long as you are visible, at least if they are paying cash or with a gift card (spouse can use credit card and the CM shouldn't get in trouble for it).
And of course if you are going to make any large purchases, be sure to upgrade to the annual pass before you do so.
You can pay off your pass whenever you want, you can also make additional payments, just know that if you make additional partial payments, it will not reduce your monthly total, just how long you will be paying the pass for.
NOTE: Maxpass with the Photo Downloads costs $75 to add to the Signature level pass.