I will have around 50 points to use in this draft.So it will be good to do combine and demo too and after that interview the rest 18 years in high range for smalls or tall players?
If you are looking only for 18y olds, use demo only and only if among 1 and 2 grey balls you find more than 10 18y old players, so you save points. If you however are looking for 19y olds too, then use also demo.
And what is good starting skills and potential for training for 19 years old players?
For 19 y olds, the minimum I'd take are 19y old SFs with 5 balls for skills and 4 balls for potential. Less than that is not worth spending your points on, IMO.