There are some fantastic ready to plant trees available now. I like MBR, and they are good value I think (an 18-22 cm oak for €24). The trees from Primo Models also look very good, although I've never tried them, and they are quite a bit more expensive at £40 for a 14-17 cm oak. I would also get stung for import charges on the latter, so I think that if I go for ready-made trees on CR Mk II then they will be from MBR.
That being said, I'm keen to try making some more of my own. I've got Gordon Gravett's book on deciduous trees on the way, so will keep experimenting. I added a little bit of horsehair to "the oak of death" this morning, using small, teased out clumps fixed on with 5-minute wood glue.
Still more to add, obviously, and then some static grass fibres to represent the smaller twigs and branches - hopefully!