Welcome biz! Good to hear from another old-school epic player.
Sounds like you have it all pretty sorted already, but here's a few answers to your questions:
1. Any basing is cool. Many people have a mix. Early on in the rules it specifies what's legal these days (pg 10, section 1.1.2).
2. Old vehicles is fine as is gluing them to counters.
3. Your counts as all sound fine. Old skullwagons as battlewagons/trukks or both! Mekboy speedstas (lifta droppas, rokits, bubble chukas) typically as gunwagonz with oddboy upgrades, others as the speedstas in the KOS list.
4. The gargant is just mad at not being in battle for ages and stomped off to find a war. He'll be back.