Hi, welcome aboard!

Get the largest tank you can afford. Mistakes are less of an impact - the more water you have the less it will fluctuate. Plus it will help you mitigate the common problem of overstocking.
That being said, a good undergravel filter powered by either a water pump or an airstone is a good idea. I would opt for the powerhead (water pump) to run it vs. the air stone method. In addition to the U/G filter (your biological filter), get a good hang on back filter as well. One that holds a sponge (mechanical) and a cartridge of activated carbon (chemical).
That way you have the three bases covered when it comes to filtration.
Start out with a snail or two and later on get the algae eater and perhaps some grass shrimp if you like. A cory catfish or something similar would be a good addition as well.
Get a good 7.0 buffer, there's some good ones on the market. That will help keep your pH where it needs to be.
Get a pH test kit as well at the very least. Your local fish store will check your ammonia and nitrate levels, those are important to check often when you first setup the tank. This way, you can keep your cost down to afford a larger tank. The pH kit should be just a couple of dollars, they all work just as good as the others. The blue drops are the one you want, not some fancy digital meter.
After the tank is up, dont scrub the gravel, just siphon it occasionally, perhaps once or twice per month, till the water level is down 10-15% or so.
Make sure you dechlorinate the water and adjust it's pH before you put the new water back into the tank to make up for what you siphon out.
No need for uv right away but there's some good inexpensive ones on the market lately should you decide to go that route.
Hope this info helps, let us know if you have any more questions, happy to help!

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