The biggest problem with a VHF-VHF repeater, is that you will effectively have one radio receiving (on 146.040 MHz, for example) and one transmitting (on 146.640 MHz), at the same time. Although they're different frequencies, and even if you're using two separate antennas, the transmitting radio is gonna interfere with the receiving radio. It can make for a poor received signal, or it can even generate enough interference to make a "feedback loop", and the repeater will never shut off. It can kill a battery very quickly that way.
You have to have some massive filtering to keep the two from interfering with each other. Most stationary repeaters use large cavity filters. Dunno about a good solution for a mobile repeater.
The simplex-recording repeaters don't have that problem; they record while the radio is receiving, then after it's finished, re-transmit the same voice recording again. They also only need one transceiver, instead of two radios. The trade-off is that they tie up the frequency for twice as long.
Cross-band repeaters (or radios acting like one) don't have to worry about interference, because the input and output frequencies are so far apart.
Finally, any device that will drive a PA speaker is an audio amplifier. There is a small amplifier built into CB radios that is used to drive the PA system. What I'm saying is that ham radios don't have that built-in amplifier, nor do most of them have a way to add an external amplifier.
Lemme know if you have more questions,
Spott