I would suggest you walk away from this horse - far too many 'ifs and buts' and its also way too pricey
I have a mare on regumate from late spring through to late autumn - she is not at all dangerous when she's not on it - the exact opposite actually, she is really affectionate but drives herself and the others totally mad when she's in season as she's 'sexing up to them all the time and gets herself kicked. I've had no problems with it but it is expensive and I wouldnt buy a horse knowing that it needed it unless it was a real prize.
I don't see why your trainer should object to you looking elsewhere, they will still benefit from your money in lessons and boarding fees.
Its always 'buyer beware' when you get a horse, take independant expert advice, always have a PPE, give the horse a really good try in the saddle and on the ground
I don't know the area well enough yet to suggest anywhere but I've heard people say good things about this place - they do lessons so you could take a look at what they have and compare prices Horses For Sale | Heritage Farm