Decide which you want, rescue or project; you may not get both with this horse. Because you are looking to buy a rescue horse specifically, it follows that you probably want to save a horse. I think that is nice. But I think you should really think about your priorities and pick a candidate which you will really help.
If you are motivated by the welfare of this particular horse, will you be helping him? If he is a complicated horse which could be made useful with proper training and you are not the talented trainer to do that, you might just mess him up and waste his chances.
Not considering the loss of your time and money, what would happen to the horse if you adopted him, worked with him carefully, and then after many months or even years of work and trying everything, the horse continued to be unpredictable and unsuitable? Would you save him? Would you give him a nice retirement? Would you be able to give him back to the horse rescue (would they allow it)? Who would be the likely buyer for this unpredictable horse if you can’t keep him?
If you really have fallen for this particular horse and you really want to rescue him then make a commitment based on the most likely outcomes. He may be a horse who will continue to have a wild soul and will never be useful to humans, in which case rescue means pasture retirement, a pasture mate to keep him happy, and years of your money. If he ends up being dangerous, that commitment might mean putting him down.
Maybe your primary goal is to have a project to ride for a few years and resell. If that is the case, then why limit your market to rescue horses? Moreover, why limit yourself to difficult and unlikely rescue horses? And really, what is a rescue horse but a horse in a bad situation or in danger of being in a bad situation? If you select a horse and are successful at training it well then you will have succeeded for yourself and for the horse. The horse will be useful and marketable and desirable. You will have fun with it and maybe make some money, and the horse will have a good chance at a good life as well. So if your goal is to train a new project, then set yourself up for success and pick a horse suited to your abilities.