Anyago

is it possible for you to take measurements of signal strength at 40m, 50m, 60m, 70m, 80m, 90m, and 100m? make a table, it might give you a more visual idea of whats going on.

There are couple of SBC's which are based on Freescale SoC, I would recommend Graperain desgin single board computer which is off-the-shelf embedded platform that consists of a compute system on module and a carrier board. The platform can be scaled up to accommodate future requirements by switching to another pin-compatible COM based on latest processors. You can choose any system on module and a compatible carrier board, to create a customized SBC that is tuned to your needs. These platforms can be used for both engineering development and mass-production.

Hiyears ago I started with a raspi and was disappointed about the performance when it comes to data acquisition and quasi rt processing. Things improved a bit with the later rasping models. Then I switched to Odroid U3 and C1 using Ubuntu on a 64G EMMC and experienced the real difference. Price wise it slightly exceeds your budget because of the 79 Euro for the EMMC, however if a 16g EMMC suits you needs HW & OS will cost you about 80 Euros. You have a 1.6 V I2C and gpio's on board that might cause additional costs for a voltage converter interfacing your peripheral devices.RegardsMartin