Ammonia, nitrate, pH and kH are pretty much essential, Ca and Cu are next. Nitrite isn't toxic in seawater so not worth testing for.
I'd rather buy decent ones for 20 dollars than cheap for 10. I'd rather not comment on what brands are good/bad in the US. Most real testing of test kits back to back in Europe has found them to be surprisingly inaccurate.
The english magazine practical fishkeeping did a bunch of test kit testing, and I think the results are on the website (google it). The results are quite scary.