Jen, I'm another vote for do it! You've got the skills, why not? I'm assuming the filly on her side would be a personal prospect? I'd go for it, work the mare, sell her on, keep the filly for yourself. Worst case scenario, the filly doesn't grow into the mare you hoped she'd become, you could always start her and sell her on. If you've got the hay & the room, I'd say go for it.
Life is like a camera. Focus on what's important, Capture the good times, Develop from the negatives and if things don't work out, Take another shot.