LoopDigger

You could try drumgizmo, you install the plugin and can download various drum kits for it. There's a metal kit on the website called someing like asiomonster off the top of my head. Then for the riffs you might be able to find some pre made midi patterns you can tweak to fit.

This is brilliant, thank you. It's been an age since I've used pure data and I'd love to get back in to it.