First off, Bless you & MFB for saving this guy from a lousy situation.
As for his costs: would your vet take payments?
I paid off an $1800 colic surgery back in the 90s in installments - it was the only way I could afford it (never regretted it either)
I've read through all 11 pages ofthis thread and I have to ask if your vet has suggested a worming program for Gus.
The prominent ribs/hipbones along with the not-so-small belly point to a worm load.
And as bunnylady said:
This is not a wild(feral) horse, just one unhandled by people and left to develop his testosterone-loaded stud self for ~5 years.
Gelding may not totally change this, and it can take months for the testosterone to totally leave his system.
Also as Gus gains weight and energy he may show you his true personality - which may or may not be as gentle as you've seen so far.
Malnourished and underweight horses can be Jekyll & Hyde once they reach full weight & muscle up.
I'm glad you have a trainer involved as even experienced horsepeople need an extra set of eyes and another couple hands to work around untrained horses.
Good Luck with your project - he has a kind eye & that says Good Things.
Sorry for the runon sentence look of this post - for some reason I can't get the text to separate into paragraphs as I type it...