If Fran can afford the 456 then I would very much advise her to go for it. I think I get what Cyclist33 is saying, but Fran is already on a budget MTB, so she will be "time served" as far as lugging heavyweight componentry round the countryside is concerned. . Why should she have to go through a series of stages of buying increasingly more expensive bikes until she lands where she will be on the On-One? As Gregg says, she will lose money hand over fist.
We must try to avoid insisting on particular budgets for beginners. Ferraris have engines and are pigs to control even for an experienced driver, but a featherweight (albeit aggressive geometry mtb) will be an absolute joy to ride no matter what the rider's level of ability.
Go for it Fran, if that's what you fancy, and what will keep you riding.