sixteencandles

It's really a no brainer! You like history, are GOOD at history and you want to do it at uni? Omg, do both! It'll give you a big advantage if you want to do His Ext later on, even if you drop one in Yr 12. You want a high ATAR and history is your best subject, you capitalise on that and make it count for at least 4 of your units. The highest scoring ATAR ever at my school did 5u of history, no maths and only bio as a science. Can be done.
PDHPE isn't PE-it's very different. Keep that in mind. It's theoretical, there's a bit of a bio side to it and other general theory. Like no pracs.

To reinforce your two main Q's
1) Many people don't find PDHPE fun or interesting as it's not what they expect. But some do enjoy it, but it is theory intensive
2) Mod and Ancient together are definitely not too much work. Proven by 1500 kids in the state year after year. And many of those do His Ext too!