I used smooth on for my casts. The key differance between those resins is the cure time and also the material strength. The 320 makes a superstong material that is strong enough that they can make rock climbing handholds out of it which can be drilled and powersanded. It may be overkill depending on what you are trying to cast.
I use the smooth cast 300 for my miniatures, on small parts it has a 3minutes pot life and will be cured with 30 min. The material sheet lists it as 15 minutes but small parts always talke longer as they don't have enough mass to generate poper heat levels.
The smoothcast 305 gives a 7 minute pot life but takes about an hour to cure on small parts. Both have pretty much identical properties as far as weight and durability the opnly differance is the cure time.
Are you looking to do transparent parts or just have the resin a solid color?
Most of the smooth on resins cure to a bright white color and you can add in dye to change the color or to give it a light shading. I use a tiny touch of black dye to tint my resin grey, thepigments are very condensed not more than a single a drop per 32 ounces. Getting the right color tone is a matter of experimenting.
I use dye made by Allumilite, I've also tried dye from micromark but it was pretty poor and made the resin really soft even after curing. I had good luck with the Allumilite stuff and had a large bottle of it so I've continued using it, I've been on the same bottle for a year at this point which goes to show just how long that stuff lasts. I think I've gone through 4oz at best and it's an 8 oz bottle. I run off a lot of casts every week, often using as much as a gallon or more resin.
Dick Blick Art has the 32oz trial size kits quite cheap, if you are going to be buying in larger volume make sure to check out
www.smoothon.com as they have one gallon units which are significantly cheaper for the volume, plus depending on where you are located you might be able to pick it up locally and save on the shipping. They have international distributors as well.