Hey there
So, props for being super keen for year 12! Really sounds like you're motivated to succeed, which is awesome
However, I don't think it's necessary to cover all of unit 3 and 4 in the holidays before year 12 starts! Sure, maybe it'll be handy to get a little bit ahead, but you don't want to get too far ahead, otherwise you'll have nothing to do at school, and you might find that you'll become disengaged or burnt out because of having nothing to keep you occupied
You really won't be disadvantaged by not being really far ahead before year 12's even started - VCE is designed to be completed within a school year
I can't comment on specific tutors, as I never had one myself, however I don't think you'll have to look further than
Tutesmart for quality tutors (without the crazy prices that other companies charge), and I'd also recommend the headstart lectures that ATAR Notes runs at the start of the year - I went last year, learnt a lot, and it was nice to go to something where I knew I was purely learning stuff that was 100% applicable to the subject.
All the best!