Kirk,
If you want to try castings, then I would, if at all possible, try to get one that is mainly bronze. They will be a bit (or a lot) more expensive, but they do tend to have much cleaner and closer tolerance castings. Ali and cast iron can be almost any shape and size.
I used to make IC engines from ali castings, and on average, I would machine up around 30% of the engine from bar stock, purely because the castings were so bad and couldn't be used. Wrong size, large inclusions, blowholes, unmachineable material, you name it, you can get it in a casting.
Try a small kit at first, so if you do mess it up, then not so much to hide under the bench.
As Stew has mentioned, the main thing is find your datum, then work from there. Every casting in the kit will need to have the datum found. Get it wrong on your first cut, and you will either be doing a rescue job, or buying a new bit. Just take it very steady and triple check everything.
John