I bought two 4 litre cans (red & white) of the 'oil based' Tremclad for $40 each. I used about half of each can, so it you were doing one colour, a single can may be sufficient.
For thinning, I bought two empty paint
cans and mixed about a half liter at a time. I added pure Mineral Spirits (not Varsol or Paint Thinner!) to bring it down to the consistency of Egg Nog. This was purely a trial and error process. The last two coats of red, I mixed thicker 'cause it wasn't covering perfectly. The final coat was thinned again as the thinned paint would allow any texture to settle out better. After about 3 to 5 minutes, the paint would lie completely flat. Note that it was *not* so thin that it would run when put on the trailer sides.
2 * $40/can 4L Tremclad (oil based)
2 * $5/can empty 4L paint cans with lids for mixing
2 * disposable paint trays (one for red, one for white. reused for each coat)
1 * $15/can 1L red oxide primer (used under the red only)
~15 ultra fine 4" foam rollers (10/pkg at Home Depot)
~10 disposable 2" foam brushes for hard to reach areas
various wet-sand paper 220, 400, 600 grit
1 * Litre Varsol used only for cleaning, degreasing and wet-sand cleanup
1 * Litre Mineral Spirits
I think that's most of it...