Hello , I'm SPM private candidate 2017. Let me answer from what I know.

1) You don't need PMR / PT3 certificate to take SPM. ( I don't have it xD )

2) Yes, you can choose any subject you like (Maximum 12 subject) except Pendidikan Seni Visual because it has coursework in school. But you also need to take six subjects that compulsory I think for new SPM candidate namely BM, BI, MATH, SC, SEJ, PAI/MORAL

3) You can self-study ( I'm also self-study at home ) if u want to know anything about how I study just ask me.

4) The requirement is you have to be 17years old at 1 January (year you take the SPM private)
For example, my birthday is on NOVEMBER, so I can't take it at age 17 because I'm not 17 yet. So I register at age of 18.
In a simple word, if you don't have PMR/PT3 you need to take it at the age of 18

For now, that's what I know. Sorry if I accidentally gave the wrong information as they change the requirement or etc.