Hello,
it's possible to tranfer your old PC as many times as you need with Transporter. I mean it's technically possible but I suppose it can possibly violate MS EULA so consult with MS support before performing it.
Then, you actually can store a VM on a shared drive but the synchronous usage won't be possible anyway - either one Mac or another.
If you need to install PD on both Macs then you'll have to buy two, because according to the license agreement, you can have one licensed product copy installed on only one machine simultaneously. So, if you wish to use our software on two Mac computers, you should uninstall the software from one machine before installing it on another or buy two licenses for each of them as I mentioned.
And as for Leopard, Parallels latest builds generally run well on it. But of course it's too fresh now so there can be some issues. But it's already a period of beta-testing so there will be an update for Parallels under Leopard really soon.
Best regards,
Alicia.