I'm 5'4" & 112lbs. I bought a new CRF150 ~2 yrs ago. I really like the size & weight but now need a little more power for the tough stuff. Speed wise I'm fine, I have no need to go faster than 40 - 50 miles/hr, thanks. I just want to climb bigger hills and rougher terrain more easily. I've just bought a big bore kit for it.
When I bought my bike there were NO used ones around at all so I had to buy new. If I was your size, given the choice, I would buy a used TTR125 (e-start if possible) and have fun getting used to biking. At some point I'd decide whether I'd sell it (with hardly any loss on the used price, I'd bet) and, if so, what was next. At least this way you wouldn't lose money by buying something new and then needing something else afterall. Moving up is a likely progression as you get better. It's all really just about whether you want to take smaller, possibly more comfortable, steps or just go for it. You need to know your personality so that you do the best scenario so as to not frustrate/scare yourself off biking or hurt yourself. As you're used to ATVing you may well be ready to ride almost any option, only you can decide. If you're really agressive and not anti-2 stroke, a KX100, or even a CR85, might work well for you. Sizewise they'd probably be perfect.
If I was buying new, I'd go the CRF150 or, if you're really strong/agressive (for a small beginner) the CRF230 and lower it. The kickstart option on the 230 is sweet, BUT (for my size) I didn't think the extra 20lbs was sweet. I drop my bike a ton but it starts very easily. In fact, I'm often kickstarted before any of the guys I ride with get their e-starts turned over.
Anyway, in case you weren't aware of the ads online for Alberta:
Calgary Bargain Finder- dirtbikes
Edmonton Bargain Finder- Dirt bikes
Of course, the best thing is to see if people with the bikes you're interested in will let you try them.
Good luck, have fun and let us know what you come up with!