chislenko

Yes, all basic technology and for the reasons in previous post I wouldn't go with it. Unfortunately cheap bikes are cheap for a reason.

20.5 inches equates to a 52 cm frame.

For reference I am 5'11" and ride a 55 cm frame. At six foot you would find a 52 cramped and small and quickly regret buying it.

I personally would never buy a bike mail order, you need to sit on it, ride it etc. Also with it being mail order are you confident with the final assembly, pedal thread direction, adjusting the barrel adjusters after a few weeks when the cables stretch. The other thing about going to a bike shop you can get him to change the brakes to UK side when he builds it.

Midgo

2nd hand one?
OLX?
Remember Portuguese seem to think they can sell second hand things for more than they paid new for it so dont be put off by ridiculous prices and dont be ashamed of offering under half price and ignore any theatrical protestations at the 'shame' of such an offer

I have found that the cheapo supermarket ones here are complete false economy, a year or so and they are destroyed and things break and/or scare you with their weakness.
I have a Decathlon one (BTWIN ORIGINAL 5) and it has lasted me well for many years. I have it set up as a sit up and beg, trekking means I can use it in the forest and on the PT roads/potholes

I'm no cycle fiend, BTW - its just a bike