if you go premix, no need to worry about the inline oil filter, and if oil pump is set properly, or failing, anytime soon.
When you remove the "seat", the number is stamped on the bottom, of it...1.5, 2.0...etc
If you never messed with it, then the seat should be 1.5. You can get an o-ring from Ace, homedepot, and replacethe one thats on the seat noe, and just re-use the seat. Needle, i guess, wouldnt be bad investment, wouldnt hurt.
When going thru/rebuilding the carb/s, DO NOT USE THE SPRINGS INSIDE THE KIT, re-use the original ones.