Can ATtiny45 be used for this application?

[system]

So PWM is probably not a good idea right?

Right.

should I maybe use ATtiny2313 with Rx/Tx to communicate with the arduino?

Good choice. SPI or I²C are also good choices.

If the ATtiny will only be reading the sensor(s) and reporting the results, you may be better served by using a 1-Wire counter.

If you do use an ATtiny, get the model with the most memory. Instead of using an ATtiny45, use an ATtiny85; instead of an ATtiny2313, use an ATtiny4313. The difference in price is generally very small and no one has ever complained about having too much memory.

Is there any major difference in complexity or speed comparing the 3 different serial types mentioned?

Which vould be the smallest/cheapest? ATtiny85 with SPI?