A new Indo-European subfamily in China

Hi, I'm new to the list.  I can't figure out how to search the
conlang archives without searching all the other archives at the same
time, so apologies in advance if this is something the list has
discussed before.  I'm inventing a new Indo-European subfamily for a
class project, and I'm going to have them migrate to China and become
a latter-day minority group.  At some point I'll sketch out some of their
syntax so I can write a tiny text in the language, but phonology and
some morphology are what I really have to nail down for this project.

It's going to be a tonal language, but I haven't really figured out
yet how languages become tonal in the first place.  Does anyone have
any idea?  I come from Singapore and my field is medieval English, so
I have practical exposure to tone through Mandarin and Hokkien
Chinese, but not much theory.  I've read that voiced consonants tend
to produce lower pitch in the following vowel, but that only gets me
a two-tone system or a pitch-accent system - I'm shooting for
something more interesting, with true tones, probably both level and
contour.  And out of curiosity, do most of the minority languages in
China have tone?  Is there a Sino-Tibetan language that doesn't have
tone?

If there are other areal phonological features that anyone thinks
might be worth including, I'd love to hear what you think.  By the
way, my subfamily is going to be separate from Tocharian, because I
know absolutely nothing about it, but I'm wondering if Tocharian
demonstrates areal influences of the kind I'm looking for.  My
subfamily is going to have internal developments too, I just want to
figure out the areal features first.

Thanks!
E-Ching