You're a bit vague. Are you going pro or doing pro-am or just really an enthusiastic home brewer?
While there are a lot of variables between systems, I start with just a few basic questions.
1) Whats the typical boil volume?
2) What's the typical fermenter volume?
3) How much grain do you use for a typical 13 or 14 plato batch?
From these three parameters, you can get mash efficiency, brewhouse efficiency, boiloff and trub loss estimates that are pretty close.
If you're setting up a new system, you can go with some typical numbers like 8% each for boiloff and trub loss, then 88% minimum mash efficiency. Of course, how the system is manufactured will have a huge impact. In that size, there are endless variations and results.
Go ahead and write recipes at 5 gallons on the system you're comfortable with. Only scale up one recipe that has a wide target range and not a lot of hops, like an American Wheat or Pale Ale. Brew it and use the measured gravity and volume(s) to nail down the system parameters. Then you can scale up the rest of the recipes with a lot more confidence.
If you're worried about ordering, you can just get a full pallet. You're going to have to order full bags of your specialty malts so just count those up from your homebrew sized recipes and order one bag of each. (Pro Tip: Standardize your specialty malts across your recipes to save money and prevent overstocking) Then, finish the pallet with the base malt, up to 40 bags per pallet. Since shipping is usually per pallet instead of weight, it doesn't make sense to order partial pallets.
With hops, wherever possible, get 11# increments. You'll have fresher hops, overall. You might want to sub out some varieties for ones you'll use more often. For instance, I have some standards I use for foundation bittering (60 - 90 min).