Okay, since this another ultra-generic question, I am going to be lazy and plagiarize myself, what I just wrote to another person who asked a similarly generic question in this very same forum.
This, somewhat edited, is what I wrote here
to someone else.
There are approximately as many ways to set up a planted tank as there are people out there who have planted tanks.
As someone said previously, the most important thing for controlling algae growth is having your light and CO2 in balance. You said low tech, so I assume that means no CO2. The size tanks you mentioned (50 or 75 gal) are too big for DIY CO2 (yeast mixture in a pop bottle), although you could do that in your 20gal. I don't think you can have angelfish in the 20gal though. You could have the other fish you listed. Remember, you can have lush, beautiful plant growth with low light tanks, but it is SLOW growth and you must be very patient.
Although someone out there has had success with just about every type of plant in low light environments, it is definitely true that certain types of plants (generally, those that grow slowly to begin with) are better suited to a low light tank. So do your research.
How do you want to set up your low tech tank? Do you want to do a Walstad style, "El Natural" tank with little light and no filters or fertilizers? Or a variant on this method, with mineralized topsoil substrate but you might have filters and fertilizers ("dirt tank")? Or get some kind of substrate designed to be beneficial for plants (there are many) and add extra fertilizers? Or you could say, forget all that, I'm going to provide all of the fertilizers myself, and go with an ultra-cheap inert substrate (pool filter sand, Black Diamond, there are many others). And I'm pretty new at all of this, so those are just the options I know something about.
Sorry. Did not mean to scare you. It is just that there are tons of options. And they all can work. So i would recommend doing a little research (this forum is a GREAT place for that!) and deciding a few things about style of tank that appeal to you. The forum is a great source of information. After you have a bit better idea of where you are trying to go with the tank, then you will have more specific questions that are easier for us to answer. And, once you get a few things set up, it is also a great place to purchase plants. Most people in the Sell/Trade section sell plants for much cheaper than other online stores. However, if you are looking for something very specific, its a bit harder to get it that way.
So that is the long answer. Now that you read it, here is the short answer:
No i do not think it would be too difficult. But before you even think about getting fish, take the next 2 weeks to plan it out, read about low tech, read about high tech, read about teh plants you want etc. There's a lot to know but if you keep at it over the next 2 months you will learn a lot.
What he said. Multiplied x 1,000. VERY important. You have started doing research- GOOD. Keep at it!