New to CNC

[JAZZCNC]

You mention using the correct tool - is there a good reference source I should go to for which cutters to use ?

Not really other than making sure you use a tool suitable for that material and any good tool provider will have info on which tool suits what material best.
After that, it's more personal choice through experience and every machine cuts slightly different depending on stiffness and several other variables. So speed n feed settings can vary widely between users.
Not many will be cutting pewter so won't get too many suggesting what they use but I'd guess any tool suitable for aluminum would work. For aluminum you want uncoated tooling, carbide works best and lasts longest, but it's expensive and with your machine you probably won't be able to cut at the correct feeds to suit carbide. In which case you'll be better with High speed steel(HSS)

Why don't you go straight on to Fusion360, it is free and much more 'organic' than SketchUp. In fact I need to learn Fusion360, I am tiring of Sketchup.

I'm not sure it's still free.? Sure I heard that they had started charging a subscription for it, but I might be wrong so don't quote me on it.!