I agree with everything that @Michael said (and upvoted his answer).
Your IDE will, by and large, be determined by your choice of processor.
Atmel are great, have been around for decades, are reasonably priced and have lots of good peripherals, plus support forums.
However, the "new kid on the block", the Raspberry Pi is now five years old and comes in several versions, plus a few spin offs.
I will just add that the OP might find things simpler with a Raspberry Pi. They have great support and a dedicated site on SE.
Starting at $5 (and only $10 including WiFi and BT), they won't break the bank. You can certainly go with C++ (which is close enough to C#), but Python is simpler and very popular.
You can also look at the other, more powerful, Raspberry Pi model's. There is a full comparison here.
You can also look at the related Orange Pi (& Orange Pi plus, etc) and Banana Pi.
Also interesting is the $9 CHIP.
There are many more, but at this point I think that listing more would just confuse you. If you want more, just Google for “single board computer”.
Think of a project first, the choose a board to suit. At current prices, there is no reason not to try two or three, perhaps implementing the same basic project on each, before making a final decision.
Choose by needed peripherals (“shields” for Arduino, “Hats” for Raspberry Pi), price and support.
If you can’t think of a project, but a cheap sensor shield/hat (like this one,
- Possibilities include: Determining orientation
- Detecting speed
- Measure environmental conditions (temperature and humidity.)
- The bright 8 x 8 RGB LED display can be used to display sensor data or games. An on-board directional joystick provides a human interface for navigation and input.
- Compatible with Raspberry Pi B+, A+, and Pi 2
And created a web page eon your PC (use Xampp). Get sensor data from the HAT using the Raspberry Pi and display in on the web page.
You might also want to read my lengthy answer to this IoT question