For my big drains like my Semi and the farm tractors, I took an old dented aluminum fuel tank (from a wrecked semi) and plasma cut the end plus about 12" off and cut in hand holds. I'm cheap and it holds 15 gallons no issue but I like the roll around drain pan you bought a lot. Just get under there and do it. Plenty of room.
Get about 40 quarts of Shell 5-40 Rotella T6 Synthetic oil and call it good, or Delo or whatever you want. I have a Cat in my road tractor (3406 E) and I use Rotella 5-40 and have for years.
My sump holds 44 quarts. I suspect yours is less as your motor is smaller than mine.
Don't forget the Allison either. You'll need an Allison Filter and Allison fluid too. Allisons like to have their fluid changed as well.
If I do it myself, I know what I'm putting in and I know it's done right.