Electronic – Best way of making a trip-wire type device

One would think the purest way is to use a photodiode to drive a transistor, which in turn drives an infrared LED. Let's call it the three parts solution (actually you also need a small series transistor for the LED). This can be done, but it has the disadvantage that everything the photodiode detects gets amplified, including noise it picks up. You don't want that.

OTOH, IR receiver modules are usually tuned to a specific protocol and frequency, but if the used frequencies are close a single receiver may be used. The module includes a filter around the center frequency, which eliminates noise, together with an AGC (Automatic Gain Control) stage, which also helps eliminating unwanted (low level) signals, like the radiation from HF fluorescent lamps. But if no real signal is present the AGC will amplify the incoming noise to a normal signal level. And this noise will be retransmitted. When a real signal is received the AGC's amplification will be adjusted, and the noise will be suppressed, so a real signal will come through properly, despite high noise levels when not transmitting.
So while there will also be a lot of noise picked up and retransmitted by an IR receiver module, just like the three parts solution. The disadvantage of the latter is that it won't suppress the noise if a proper signal is detected. Its advantage is that it's protocol-independent.

As for power, this is a tall order. The three component solution will transmit noise all of the time, and will drain the battery fast. The receiver module contains more electronics, and will draw around 0.5 to 1 mA. Which is also too much to let the battery last six months.

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If the power is really premium you may have to stick to a specific protocol and decode the received commands before retransmitting. The \$\mu\$C will consume some power too, but will cause the LED to remain off for most of the time, like up to 99.99%, since it won't transmit noise, just valid commands. It's possible to program the \$\mu\$C to decode multiple protocols, but like I said earlier, they'll have to have close characteristics, like pulse-pause ratio and carrier frequency.

You can build a simple RC5 decoder using a PIC microcontroller.

Transmitter: Use a universal remote with Phillips TV settings. RC5 is a protocol designed by Phillips for their TV, VCR, etc remote controls. By using RC5 protocol, you eliminate the need to design your transmitter. Just use any universal remote control and program it for Phillips TV. I have seen universal remotes floating at 99-cents only stores in Southern CA.

Receiver: A RC5 decoder can be built around a PIC microcontrooler. Take a look at RC5 Decoder with PIC Microcontroller. This article describes RC5 protocol, hardware and software needed to decode RC5. You many have to change the code to your taste. To drive motors, use simple transistor drivers at PIC outputs. The motor driver depends on the type of motor you are using.