During the cycle you should test for ammonia, nitrite and nitrate. Afterwards you will need to test and adjust your calcium and alkalinity levels.
Phosphate test kit is good to have as well.
If you want to skimp, skimp on the ammonia and nitrite kits, you wont need these after the cycle. I would not skimp on the nitrate, calcium, alk, phosphate and ph test kits as you will use these often. Salifert is my favorite brand. Easy to use and easy to read results.
Oh and a hydrometer, if you dont have one already.
If you want to spend some cash wisely, I would invest in a ph pen or monitor and a good refractometer with temp compensation.
If you plan on adding any supplements (ie magnesium or iodine) make sure you have a test kit to test your levels.