Hello! I work with a bunch of freelancers on Upwork.com so you can pay them directly through the platform on a flat rate or hourly rate. If this freelancer gets sponsors for you, you can give them a percentage of the sale as a commission. Sales reps usually receive somewhere between 10-20% of the sale so you can use that as a gage for your freelancer and how much money they will be bringing in. For flat rate you can do hourly or project based on how many hours you assume this freelancer will work. If you want to further discuss feel free to set up a call!
1-If the deck was to be used by her/him, then it shouldn't be paid for, since it's a tool they need it. If you ask for it as something you will use too, then $300-500 it's more than enough (if it's good)
2-Sponsors as in investors? Investors don't like people that charge a commission for fundraising (FYI). If it's sponsors as in clients or corporations, then commission.