bare minimum is a SeaChem brand Marine Basic kit. ($25) It has 2 ammonia tests, nitrate, nitrite, pH, and alkalinity. They make a good phosphate kit too, but it's separate($10). Since you said 'reef', you'll also need a calcium test kit, like SeaChem's Reef Status - Calcium kit ($25). Its a little fancier, very accurate...more akin to the Salifert line in quality and price.
Salifert is a great brand too. A little pricey ($20 per test kit) but the quality is top notch. I use their Magnesium test kit ($20)...the instructions are a little weird, obviously a poor translation, but its usable.
Former advisor and planted tank geek...life's moved on though.