Dual power supply confusing TTL serial?
I would start with the 7 V to 5 V power regulator: there are zillions of regulators for that. You need an LDO (for Low Drop Out). If 500 mA is enough the LF50 is a good choice: drop out less than 0.5 V, ground current less than 0.5 mA. 2 % or 1 % accuracy.

As I understand you from the comments that infamous "cable" is a 50 cm wire, right? Then there should be no need for the two transistor inverters since the supply voltage for both microcontroller and TTL is 5 V. An AVR output can easily drive a TTL port half a meter further. You might replace the 74LS00 with a 74HC132 Schmitt-trigger input quad NAND if it's otherwise too noisy.