The best package deal to buy of fertz?
"Best" package really depends on what you value the most: best value for $ spent (dry ferts), ease of use (debatable), ease of storage (liquid), etc. Buying 1 lbs. bags of dry chemicals will cost roughly the same (maybe less) as buying most of the SeaChem line of liquids, the difference being that the dry ferts will last you a very, very long time on such a small tank. The problem is if you're only using them on a 5.5 gallon, they will start to clump together over time as water is absorbed from the air (hence the storage problem). For dosing small tanks, dry dosing can be done with tiny measuring spoons like these
and liquids can be measured with a syringe if you want to be precise.
The essential fertz I should purchase?
KNO3 - Nitrate
KH2P04 - Phosphate
CSM+B - Trace
K2SO4 - Potassium
"GH Booster" - (Calcium, Magnesium, Potassium, and Iron)
If you have hard water you don't necessarily need GH Booster, but if you don't use GH Booster you should use the K2SO4.
You can also get "root tabs", and use them in conjunction with your water column dosing regime as a useful back-up. That way plants have the option of drawing what nutrients they want from where, and are even less likely to run out of what they need.
Routine and waterchanges will depend on whether your tank is "low-tech" or "high-tech". Low-tech tanks need less ferts, and few water changes; high-tech tanks benefit from daily ferts & weekly large waterchanges. Example: For a High-Tech 5.5 gallon tank using dry ferts you would add 3x a week 1/16th (or "pinch" spoon) of Nitrate, and a "drop" spoon (1/64th tsp) would be the 3x weekly dose of Potassium, Trace, and (on alternate days) Phosphates.