If the output power is low and you know programming, then you can make such a device for under $30 in parts, if we don't count the suitcase.
I made a device which is nearly the same as you need using a TI Tiva C launchpad for 13 USD and a AD9850 board for 10 USD. There are numerous similar boards on eBay and related sites, so you could get one for even less, if you shop around. You can replace the Launchpad for a similar board, such as for example STM32VLDISCOVERY or something like that.
Basically, the Launchpad has an analog to digital converter that I use to sample the audio. In my particular case, the ADC's internal reference is set to 3 V, so I used a 3.0 V voltage reference and a resistor divider plus a capacitor to move my audio input so that it's DC value is 1.5 V.
In the launchpad, I have a loop that samples the ADC, waits for the sampling to complete and then based on the amplitude of the signal calculates the deviation.
The ADC has 12 bits of resolution, so we can expect values from 0 to 4095. Using a simple proportion, you can scale that to the range of -deviation to +deviation for your band. What I did next was to add the deviation to a fixed frequency which represents input when the ADC provides zero.
Next step is to output the data to the AD9850 and set correct output frequency. A problem here appears if you want to use frequencies above around 40 MHz, due to the limitations of the chip. There is a way to work around that, by using harmonics of the output and a bandpass filter to just let the frequency range you want out. The procedure is explained in Analog Devices application note AN-543: High Quality, All-Digital RF Frequency Modulation Generation with the ADSP-2181 and the AD9850 DDS.
Using this procedure, I was able to output high-quality mono audio signal to an FM radio with very little effort.
The main advantage of this approach, compared to using an analog transmitter is that on the hardware side, except for the output filter, you don't really care about the frequency band you're using (as long as AD9850 can produce required frequency) and you don't care about the deviation, since you can set it in software.
As for the power supply, m solution requires +5 V and uses less than 100 mA and is normally powered from a USB port.
For the antenna, very little is needed. For example, just plugging the AD9850 module I got into a bread-board was enough to cover big part of my room.
In the end, be sure to monitor the output of your radio transmitter to be sure that it only outputs in the band in which you have a license to operate! Additional filtering will probably be required for that.
