What are you wanting to do with your horse? Just mosey down the road/trails? TWHs are great for that (there's a group of... uh... mature... ladies, known as the Wild Bunch, who zoom down the trails of a local park on their TWHs. They're a fun group but hard to keep up with!)
If your trainer won't help you look at horses, why should her opinion count? She hasn't even seen the horse, so how can she say no? If you buy the mare, will she refuse to give you lessons? Personally, even if I loved the horse I was taking lessons on, once I bought it, I wouldn't want anyone else riding the horse but me. What happens if you want to ride and your trainer has a lesson scheduled? Or the horse has already been worked all day and you'd like to ride too? Not fair to the horse, I think. I don't think your trainer needs a year to replace the lesson horse either.
Don't you have a friend who's experienced with horses helping you? (or was that someone else?) Anyway, if so, what does she think of the mare?