Yes a snow plow package will raise the front of the pickup. You probably will not need any extra capacity in the F-250 to carry extra weight, and the extra stiff front springs that become part of the snow plow package can just make the ride stiff in your truck.
You really only need to have about 2,000 pounds of cargo rating. The 17" diameter stock tires are what I would desire. The 18" rims would make it run about 1/2" taller, because outside diameter of the tire will be higher. 20" or other larger rims make no sense to me, as they are 'low profile' and have less space between the rim and the road. That 5" of rubber is important in absorbing pot holes and vibrations!
Do not 'settle' for a truck that you do not want! I ordered the color that I like, all the options that I wanted without any extra options that I do not want to purchase. You still have a great truck, make sure that the dealership thinks you still love that truck, and the only way they will make a sale is to give you top dollar as a trade. You already know that the truck will be up for sale at a huge markup as soon as your deal is done, and the 3 days have passed so you do not back out of that deal.
If they have the right color and a couple of extra things at their dealership or a nearby one, fine. Otherwise tell them that you will be looking at the GM dealership too (you do not have to go there) and they might come down on the price a little bit.
Download and understand the towing guide. IT will tell you the base weight of many pickups (look on the page stating max cargo rating to carry a cabover camper). Also check the tow rating for gas and diesel engines. I think my next truck will be gas, the mark up of $8,000 to get a diesel is way to expensive!. And the truck is lighter, will carry slightly more cargo, and mileage is not that much worst.
Good luck!
Fred.