I would rather go with ESP8266. You can program that from the Arduino IDE. You don't need anything else than a programming cable (an USB-UART adapter), and you can connect the force sensor to the analog input with one pullup resistor.
There is a similar idea with one button here:
You can build your own force sensor:
If you don't really need to measure the force, there are much easier solutions to detect weight of a person. These give a simple contact once pressed, and can be used like a button (see the first link in my answer).
http://learning.media.mit.edu/projects/gogo/documents/making%20sensors.html#Paper and Aluminum foil.
It is not really possible to buy everything as one batch :) I would say you need:
I suggest you to find an ESP8266 based tutorial, Adafruit has a lot of those, and then you'll have an understanding about the tools and components needed.
If you buy some Arduino, consider buying the ones using 32u4 chip, such as Arduino Leonardo or Arduino Micro. The 32u4 chip has more memory, and more performance than the original Arduino Uno, plus it can work as an USB mouse or an USB keyboard too.
For infra (do you mean a PIR which detects humans, or a reflective sensor?) you have quite a lot of possibilities. In case of using ESP8266 directly, check that the sensors work on 3.3V. Some stuff works on 5V, others on 3.3V. You can interface them easily, but this is just another extra thing to cover, so better go with a dedicated 3.3V sensor.
Ultrasonic sensors are coming also in a lot of varieties. Do you need something ridiculously cheap? A sensor with a short range? A sensor with a longer range? I think you have to set up your goal first.
The Leonardo and Uno and Micro boards work both with 5V and 3.3V sensors. Driving these sensors are quite standard, you need to power them with two wires, and have one wire which goes from 0V to 5V (or vice versa) when something is detected. Ultrasonic sensors vary: there are sensors with one analog output, giving a voltage between 0V and 5V propoprtional to the distance. However most of ultrasonic sensors are simple as the ultrasonic principles: they have one input what you drive, and will give one output of the recived echo signal. In this case you have to measure the time elapsed between the drive signal and the echo, which depends on the distance of the object from the sensor.