Hopefully I won't have to buy anything other than maybe a trace solution?
A trace mix (mirco) and NPK (macro) Nitrate, Phosphate, Potassium or go with an enriched substrate type tank to support the tank weeds.
There are literally endless ways discussed in these day's of the internet information exchange to 'method' a planted tank. My personal choice is to use a potting soil type dirt mix as the primary base with an inert capping material. This is by no means a good choice for everyone and has a number of benefits but also compromises.
Going low tech or not I like dirt tanks and the rewards and trade off considerations suit me. Low cost and an easier maintenance schedule are fair to say across the board after the tank stabilizes.
Diana Walstad's book ended up being a must read and I added it to my library. Hopefully most are reading enough information first and thinking it through. Those that don't are the main group posting horror stories about using soil as an aquarium base.
Two HUGE considerations doing this.
Using 'natural' soils READ the contents on your bag of dirt! I know it contains dirt,,, (duh),,,
but NO COW POO! chemical ferts or wetting agents Small amounts of chicken waste or worm castings can work great. I avoid cow manure but others have achieved nice stable tanks using it in the mixture.
Also remember PLZ that while natural tanks (dirt base) and seeded filters can be stocked from day one go lightly with your first stocking list. Dirt goes through changes going from dry to saturated (submerged) and the rate of break down on the organics changes too. Sometimes it can be more than the tank and fish can handle.
For the first couple of months whether you want to or not test your water. Every couple of days and be ready to change it if the soil burps (it can happen). You might have a tank like mine that ran straight through the issues quickly and was trouble free from then on. Lots of plants (including floaters), no hard scape to trap the soil gases, control the light (a big key to dodging algae), watch things and let the tank settle (month maybe two). The capping material needs to be small enough to contain the soil yet allow the gas exchange to occur.
Attention starting out, more or less high maintenance in the beginning. Things can get busy if a bump in water parameters occurs. All the organic material and the bacteria that chew through it do give you free CO2 for a period of time.
The second major trade off you make is that rooted plants are there to stay. Removing plants with a good root structure is a HUGE PITA. I had an Amazon Sword that had to go. Cutting around the root ball directly under it I killed the plant taking it out and left all the root runners in place. Thinning a field of crypts means a water change and repairing the cap adding more material. Soil tanks are a set it and forget it type of tanking (imo). If you like to change things around, re-scape, swap out plants then NPT is not for you. If you want to top off the tank when the water gets low, trim to make room for the fish to swim and not dose for months it might be what your looking for.
A planted tank is more than a basic water, fish tank. If you want to tank plants they need to be fed if the system is to succeed.