Alright… so here is my sum up:
Bourns EM14R0D: Great encoder. It runs very smooth. It has a switch integrated which felt great at first glance. But when looking at it more detailed it doesn’t give a “defined” feedback if the switch is pressed or not. About 27€ / pc. Too expensive in my opinion…
Bourns EPS1D-F19-AE0024L: Also a great encoder and with 6,50€ less expensive. Also the switch feels great because it is not tactile. The encoder-shaft moves about 2-3mm into the housing and gives a nice feedback. It runs also very smooth. But: It has a big housing. Its about 20x22mm. And the biggest drawback: It’s shaft is only 4,7mm thick though you can not use standard knobs.
Bourns PEC09-2020F-S0015: Only 2,75€ / pc. It feels okay but feels way cheaper as the encoders mentioned before. It requires more torque to start turning the encoder as on other encoders I’ve tested (like PEC11L). The running-torque seems to be the same as on PEC11L. The integrated switch is standard like on any other encoders. Nothing special here.
CTS 290VAB0R201B1: Do not buy it! It costs little less than 4€ but it feels totally cheap. It requires almost no torque to turn the encoder.
Bourns 3315R: If you are look for very small encoders without switch and great running torque, you should take a look at these buddys. The feel great, but are miniature. They have a very small shaft. But it gives you a nice potentiometer-feeling.
I think I’ll go the step you recommended above. I will try the sin/con-pots. A friend of mine is able to write the necessary code for a small MSP-IC. So this may work out very well…