Well, Im quite new myself, but I'll try to help out.
A tumbler is not a must. The water with vineger(sp?) will clean the cases. It wont really poilsh them though. You can buy a tumbler from MidwayUSA for 40-50 bucks, and the corn from a pet store for about 2 bucks per 4 pounds.
A set of scale check weights is a good idea as well. Especially if you buy one used.
A cartridge headspace gauge might not be a bad idea either.
Powder? I dont really know, I only shoot rifle currently. I *think* that Unique would be a good choice though.
Primers? You can go with Winchester Large Pistol, Federal LP, CCI...
Bullets? Regular FMJs should be fine for your applications. However, hard cast lead bullets might not be bad for just plinking. You can save some more money this way.
One last thing, good choice on the Dillon. I dont own one yet, but probably in less than a year I will be upgrading to an RL550B.
Sorry about the long post. Hope this helps.