That being said, thinking logically, I would have to tell you to pass on your lease mare (as adorable as she is).
Should you buy her now( at a higher price than you were wanting), you still have to deal with boarding her until your plans are complete, and you have your own place. (And what if your plans fall through? Or are delayed? Things happen.) Are you prepared for that?
Also, you've expressed interest in an activity that she is not good at. While there is a good possibility that she COULD become good at trail riding, there is always the distinct possibility that she also never will be a trail horse. Are you okay with that?
I struggle sometimes with my mare (Now 24, but believes she is 5). She is what would be considered "bomb proof" and nearly child proof and just the best all around trail horse you could get. However, I was looking at possibly doing some local shows with her. This would have entailed her going to my trainer for 30 days while he worked with me and her.
I had never seen her pin her ears at anything but my other horse before, and never like this. As soon as she and I started working a bit in the arena, her ears were plastered to her head and she was acting up. I wasn't asking for anything too difficult, just trotting on the right leads in a more collected frame. Definitely not super collected or anything, but a nice little frame.
After not too long, it was very apparent that what ever training/showing she had done years before I got her, had left a bad taste in her mouth and she would just never be happy about working in an arena setting.
Was it disappointing? Certainly. However, I know that it wouldn't be fair to her to push her to do something she just does not like, when she is good and happy at something else. I have to be fair to her. I have high hopes for my young gelding, but that is a long way off yet. Will you be able to set aside your wants for her needs if it comes to that? I'm not saying that we should let old horses have their way on everything and that they don't need discipline, just that they have done their time, and (within acceptable limits) it's not really fair to them that they aren't what you need.
I'm sure that all could have been put a lot more eloquently, but I spent today in the hot sun, so I'm a bit fried! LOL. Good luck with what ever you choose!