You have already a starter kit and want to do cooler things? Like doing real work giving commands trough wi-fi, discover cutting edge technology and drive robots? but still on a budget?
- Wemos D1 mini
This is a very compact and powerful microcontrolle; 160MHz, built in WiFi, 1Mb memory and 50Kb ram.
- ADS1115 16bit adc
to add 4 pin with high quality 16bit analog pins for about 0.5€ each
- Shift register
each one of these adds 8 digital pins for 0.09€
- Battery Charger Shield
A shield for the Wemos to add battery management and make very compact wireless devices
- Lipo cell 1s
LiPo cells are the most dense batteries. Holds about the power of a 9v alcaline battery, but it is way more efficient, light and rechargeable.
It is an I2C 3Axis accelerometer, 3axis gyro and 0.01°C temperature sensor.
- Mini OLED screen
0.66″ 64x48px OLED screen. Compared to LCD they are very bright, use very low current.
- Motor Driver dual 12v 2A
This is a very cheap but reliable DC motor driver, which allows direction and speed control.
- Sd card shield
SD cards are the cheapest way to add bulk storage. Especially useful with the ESP8266 as you can then use it as a web server, with video and images.
In total you would spend ~23€, for a lot of parts that will teach you modern techniques.
I2C and SPI allows to add many peripherals to the main controller, using only a few pins and low latency.
ADS1115 and shift registers allow to add more pins as needed, beating the need to use a much more expensive microcontroller like the Due or the Mega.
WiFi is the most ubuquitous network protocol, and by far the most flexible. With this you can make an IoT thing and connect directly to internet. Telegram for example is very easy to use trough a bot managed by the ESP8266.
Cheap and popular parts allows also to eventually make prototype for products. and have lots of support around the internet.