Have you tried a regular Tamarillo before? There's a bit of a phenomenon where it takes a few tries of the fruit to develop a taste for it. From the descriptions I've seen Forest Tamarillo is a very similar fruit. Tamarillos are largely frost intolerant but they are usually planted in the understory of bigger trees or near a building, giving a microclimate.
When they are mature they would probably take some light frosts, now Forest Tamarillo could very well be hardier, so I would say as long as you have a microclimate that has natural protection from frost, you should be okay once the Forest Tamarillo has a good trunk on it.