Hi Aquaria central

So today I agreed a price for a Juwel Lido 120 from someone nearby and I'm really excited for my new purchase! Just wanted to get a bit of advice so I can get the best from my new setup.

I have a little experience with freshwater aquariums as I had a few at different points growing up but its been about 15 years since the last one. I remember being overcome with excitement when my little guppies gave birth and spent my time staring at the hatchlings in their little growing chamber. Those were the days.:nilly:

Anyway I was hoping to get as much enjoyment out of an aquarium this time round but thought I could be a bit more ambitious. I've been doing a bit of research and I think I would be happiest with a reasonably low maintenance planted aquarium. The lady selling me the tank is supplying it with its current gravel and some plants: I think there was some Java fern and some form of anubia in there. They were growing fine under the light currently with the tank so I will carry on with that setup, I don't fancy getting into CO2 and all the rest of it. Not just yet anyway. She said she will also leave 10-20% of the water in there when she empties it so the natural bacteria remains. Whats the best way to test natural bacteria in a tank and the hardness of water in my area? maybe there is a decent article someone can link me to?

Now as for stock I remember seeing a fantastic tank in a shop once which had about 30 Neon tetras shoaling perfectly, I'd like to re-create something like this but am not sure how to get them to do that or whether I would have better luck with a different species, perhaps harlequins of some sort. Really I just want one or maybe 2 big shoals in the tank that sometimes merge into one bigger shoal, any other stock would be for maintenance. So a pleco of some sort and then I have read about invertebrates which are good for cleaning, I think a shrimp of some variety would be nice.

So: Am I making any grave errors or are there any top tips before I spend more money?
How many Tetras/Harlequins in a 120ltr Lido to make a nice school and how can I get the fish to school tightly?
Is there a good type of pleco you can recommend?
A nice shrimp or crab for extra housekeeping?

Any help is greatly appreciated and I will of course add some photos/Youtube vids of my success or failure if People are interested.

Thanks
Dan