On a Pi2 the display will have no impact whatsoever on the availability of GPIO pins. It uses i2c-0, which is reserved for the GPU to talk to the camera, display, and HATs.
Pinouts - stick with the Pi2 and refer to http://www.element14.com/community/docs ... ock-pinout
That should be standard between Pi2, A+, B+, and Zero, so that covers all the mainstream Pi flavours.
Minor potential for confusion in that I'll refer to GPIO number which is the GPIO pin on the SoC, not the pin number on the header. The diagram quotes both anyway. (wiringPi does have a different numbering that is unique to itself, but can also work with SoC GPIO numbers).
For I2C you want GPIOs 2&3, which are pin numbers 3 & 5 on the 40 way header. Don't forget to include a ground between the devices as well. Those pins already have 1k8ohm I2C pull ups fitted, so you don't even need to worry about those. Just don't forget that the Pi only works with 3.3V signals - feeding 5V into GPIO pins will probably kill them.
From what you have posted of wanting to talk to an I2C controlled amplifier, FM Tx/Rx, and display, that is all perfectly feasible on one Pi. Unless you need the physical separation don't waste time on splitting it into 2 parts.
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