People are telling me to buy this one horse..?

Ok we currently have 3 horses and thats all we are allowed at the stable we board at. 2 people have already told me to buy this other horse at our stable cause Im riding him and helping train him they say he would be a good match for me and be a good project. And he seems to really trust me cause yesterday this guy tried to ride him and he reared up twice and the guy fell off(before that this trainer was riding him and he broke the trainers finger) and after the owner lunged him a little and I hoped right up and he was perfect. They say hes a better match height wise for me(Im about 5foot5inches and my current 2 horses i can ride are about 15hh and our colt should mature to about that size too but the horse that they say i should buy is like 16hh if not a little taller). The lady said she would give me a good deal on him, shes currently asking $3500 but everyone says hes not worth that. Hes a 4 year old Appendix QH Hes got very nice gaits and his conformation is preatty good though he does seem a little east/west in his front feet(though I think its the way his hoofs are trimmed). Were sure he was beat by men before cause he doesnt like them at all. What should I do? Im trying to get my mom to let me sell our mare to this other lady across from us at the barn who wants a horse for her bf cause our mare is perfect for beginners and is a great trail horse and thats all she really wants for her bf. But my mom doesnt wanna sell her cause she has foundered before and she needs special trimming(which I do myself) So How can I persuade my mom to sell her?