As regards number 2, you don't need anymore then you start with for a large proportion of the game that you won't pick up anyway and a horse deed is wasted cash (you get a top tier horse pretty early, just don't lose it and if you do you can always find another). Spend your money on some items like the bandolier for double ammo capacity, but you likely won't need more then a bit of hunting to afford such goods which you'll be doing anyway.
As regards 3, I actually pretty much stopped hunting when I hit Mexico period, I was getting enough money and ammo from missions and I had no need to spend, infact other then my Buffalo rifle I can't think of anything I purchased from a store after I made it there. Though you may be able to make more money selling Mexico, you should have enough to get by if you shoot plenty of stuff early on regardless of where you sell it. If anything you need the money earlier rather then later when you hit Mexico. If you're in Mexico sell there, if you haven't made it there don't hesitate to sell your stuff anyway. I don't even know if you get more money in Mexico, because I didn't need to make more money then I did in missions by then so I never noticed.
Basically, hunt early as and when you can, just to get a bit of money in your account for smaller purchases, but you don't need to purchase any weaponry for the majority of the game you are given what you need when you need it. So instead of grinding your way trying to make extra money, just don't buy weapons and ammo unless you really need it, I was playing on expert and I never needed to buy any weapon at all. If you really like the look of something by all means go for it with bounties and other fun stuff, but I have £3000+ and over £3000s worth of gold (treasure) I could sell at the end of the story as I sit. It's not because you get a stack of money towards the end, I had £1000+ sitting doing nothing for a large part of the game.
It's not a particularly difficult game for the most part, even without Auto Aim on, you don't need any special tips really even the ones I've not crossed out aren't a big deal. Take a little time to familiarise yourself with the horses and work out how exactly they work (don't just mash sprint then wait for it to fill up to max again, pace the horse well and use stamina steadily, then bring it back up steadily rather then just wait by tapping X a little as it recovers). You don't need money for the most part so just save it up from what you kill as you travel and buy the things you like the look of, maps are nice to have though.
. It's so easy to get lost in the old west just doing so many things.
