Good questions and well organized.
1. It isn't harder to haul 3-4 horses really, but can the truck do the load? Gas mileage drops, speed in merging drops, etc.
2. We researched trailers about 15 years ago and there was actually a study done in Great Britain about what the best horse hauling condition was and without going into all of the details, a stock trailer won hands down. And horses untied. I know all of the slant loaders will not agree with this. But someone doing a doctorate rated horses condition, respiratory problems, etc. from different conditions, and open stalls and movable positions were best. So that's what we got.
Very few horses will not load into an open stock trailer. I have loaded many problem horses for other people in a stock trailer and let them loose to establish their footing and look around.
Other advantage, a stock trailer can be used to haul lots of other things: lumber, hay, furniture, etc. And they are the most reasonable.
Downside: weather coming through the sides...rain, snow, etc. I blanket my horses with waterproof blankets. I also use fly masks for debris and bugs flying in.
Plus: open and breezy in hot weather.
3. I am not a fan of ramps for these reasons: you always have to check those hinges for stability and try to find a pretty flat place to load. If you have a problem loader and are alone, how do you get that ramp up? With a step up, you can walk on, turn around, and close the door with the other hand. If we have a horse with a problem learning to step up, we back the trailer into a slight hill to drop the rear of the trailer to almost ground level and teach them to load in increments.
Since you have such good questions, I am sure you will research all of the other factors that make a safe trailer. With this budget, you should find something decent that may need some new tires or a coat of paint or a floor replaced.
When you learn to haul, do not bypass electric brakes. Don't buy a trailer without them and get the unit installed in your vehicle. Take the trailer out without horses and practice driving with it, braking with it, and merging into traffic with it and BACKING it. Happy hauling.
Edit: Our stock trailer has a middle divider to haul them compartmentalized and a center divider for the front two if you wish to haul them like a regular 2 horse. We put two horses in the back that get along well and one pair used to both turn around simultaneously and face the rear and never got tangled up.