What casting process do you plan on using? That will partly be dictated by the design of the part and type of finish you need. Once that is established, that will then be able to identify the possible choices of materials for the tooling.
Are you the caster or do you need a foundry source? If you need to find a caster, they maybe able to recommend some tooling material that have had good lick with in the past for the volume you need.? Plus they should be able to work with your machine shop up front to establish the runner, gating and riser resign into the tooling.
If you have several thin wall that will need to feed thick wall sections then you would be looking at possibly a HP Die Cast process to allow the metal to feed the thicker section before freezing off in the thinner sections. Then you are talking $$$ for tooling, such as H13.
If the design and finish will let you can get by with using sand cores, there are a couple different materials you could use. I would recommend "Pattern Plank - PP1052" which would hold up for the volume you are looking to cast.
For a sand cast, all thickness is fairly constant and thick enought to allow good metal flow without freezing off to soon, and draft is 5 degrees or greater (to lessen tooling wear). This would also be the cheapest way to go.
There are a several different versions of Pattern Plank or Rein Board available that could also work. You may want to check with the suppliers for specs on the products. The "Pattern Plank - PP1052" (red stuff) cuts fast and clean and if you do need to do a fix, that can be done using an epoxy resin that cures within minutes. I've used this on core boxes used to make a few thousand sand cores.
Hope this helps.