Welcome to OCC!
I would recommend a good 30 weight low ash oil. CaseIH has one as does Kohler. I am sure there are others. I haven't found any oil that costs near $100 per quart. Good oil, regularly changed, is economical in the long run.
Personally, I would use CaseIH HyTran. We know that works. I guess any tractor fluid that meets the IH B-6 specification is alright. A CaseIH filter is good, as are those from Wix, NAPA, Baldwin and Fleetguard. Be sure to use the correct hydraulic filter part number and not a standard engine oil filter that bolts up. There are backflow issues with an engine oil filter.
Your engine calls for a Champion Spark Plug #RBL-154. You can use any brand that meets that heat range number. I like Champions. They are available nearly anywhere, NAPA, local parts store, Lawn and Garden centers, and hardware stores. They are reasonably priced. Get another set for spares.
Somebody who knows fuel systems and carbruetors can answer that question for you. Ask in the CCC tractor section. You'll get more answersthan you need. I don't know the answer.
Blades can be had at a Cub Cadet Dealer, or lawn and garden center, or Tractor Supply. Oregon is a good after-market blade. I hope they make them for the deck. Try a set of gator blades. I like mine.
Read this:
http://ccmanuals.info/pdf/682-782%20...s%20Manual.pdf It has a lot of useful information in it.
Most of all, enjoy that 682. It is a great tractor!