I have some experience with FDM printing and many casting techniques. Don't want to bore you or write a book, but I can take a stab at any questions you have, and I'll tell you what I don't know.
Few (semi-)pro tips randomly pounded out on a work break:
If your machine will print ABS, ABS is far stronger in use than PLA, but PLA will likely have better cosmetics as-printed.
It's not uncommon to to print ABS, then do high-build primer / sanding and finish with a good paint for an impressive finished product.
If you're printing foundry patterns to ram up sand molds, print it hollow, then fill the inside with epoxy or epoxy-based filler (Bondo is fine), then bodywork as above.
Large parts will have a boatload of sinks if cast solid - look at the ribbed structure of cast mechanical parts --> that's the ticket. Same for plastic parts: non-solid with ribs can give plastic parts equal or better strength than fully solid plastic parts while saving a chitload of material.
Printing wax for investment casting works awesome, but learning investment casting from scratch could be . . . frustrating. Local colleges or independent art foundries may take on investment casting jobs with your printed patterns. Also, investment casting opens you up to aluminum, brass, bronze, even iron or steel (but likely not at the school or art foundry level).
If you're home-casting aluminum, degassing is critical unless big porosity is OK or you set up your mold to put most of the porosity in the sprue or riser(s), or non-critical areas.
If something is equally easy to hog out vs. cast, machining is the winner 9X/10.
For high-detail aluminum parts, rubber-plaster casting will generally kick sand casting's ass for fine detail definition.
Cast parts with sharp corners are bad - run the biggest fillets you can live with.
Avoid thick sections in castings - you will get sinks, larger porosity, and shrinkage cracking in thick sections.
Gravity casting placards / emblems: put your presentation surface down so the porosity is on the non-presentation side (air bubbles rise).
Zinc is better at filling fine mold details vs. aluminum, and easier to cast due to lower melt temperature. If you don't need the light weight or strength of some aluminums, zinc is a good option.
Moisture can kill you! - if adding metal to your melt, make sure it's been preheated to drive off the moisture, or a steam explosion could ruin your day.
Dammit, I'm already writing a fookin' book
That was stream-of-consciousness, no particular order - sorry.
I'll shut up now, but let me know if you have any questions.