The auto trans fluid interval on the Elantra MDs w/1.8L engine is 120K. The dealer is equipped to do it and it is not a DIY job (speaking as someone that has pulled engines, replaced gas tanks, etc.) The procedure is quite focused, requiring certain temperature(s) and such. There is no transmission dipstick from which to add or draw out fluid.
Believe it or not, Amazon and WalMart both carry synthetic engine oils that rival the best of the best for quite a bit less cost. Translation: the company(ies) making the oils are the same sources for very high end synthetic oils, and in head-to-head testing you can look up on the InterWeb, perform at the same levels or are just barely edged out by far more expensive alternatives.
Iridium spark plugs are likely to last the longest while operating at a high level of performance the whole service period.
You may wish to invest in an oil-less, reusable air filter.