Alan's right there is plex support, there's also nothing from stopping you from installing additional non-amahi packages/programs (like Plex, minecraft servers, you're preferred torrent downloader, e-mail server just to name some of what I have) on Amahi as well.

You've got a couple questions here:

I haven't done it, but it should be a matter of running (as root) greyhole —fsck -o

True but DGStan would also have to have something send the Amahi the magic packet to have it wake, depending on what client they're using to view the stream that may or may not be easy to do.

I'm sorry you're right the Amahi app is in the tester program, because I wasn't I went and got the regular Plex install packages (I'm running Amahi 6 on Ubuntu 12.04, the newest Amahi is 7 on Fedora) and had no problems running configuring the standard Plex server on an Amahi server. That said I don't know how…

I use amahi to access my non-multimedia files away from home just as the article describes, then for multimedia I run Plex (which has an amahi app for the server side) to stream to a Roku in the house or my other devices away from home. it eliminates the need for a PC by my TV at all because the Roku runs Plex so my…

Not using amahi for DHCP and DNS certainly would. (Since the amahi server wouldn't/couldn't be running while windows on the desktop was booting, if the amahi server were your DHCP server, your desktop would never get an IP address)

I don't care for either of those solutions simply because you're limited to a set number of hard drives, with my amahi setup, and 2 esata multipliers I have 16 drives connected to my amahi server and could add more.

As long as the printer is supported by linux it's fine, I've had as many as 5 printers running through Amahi without a problem.

You can do this with some modifications. since it's in a VM you'd want to turn off having Amahi be your DHCP server and probably also your DNS server since it obviously wouldn't be up and running when your desktop is looking for a DHCP address at boot time.
You'd have to make some modifications to the VPN config files…

if all you're looking to do is VPN and/or file serving your specs should be ok (though if you could find more ram that'd be better)

No problem, for strictly file serving in a home environment anything made in the last 2-3 decades would be fine (many people use raspberry pi's because of the lower power consumption that seems to be one of your driving factors). If you were getting into a small business, or you're moving a crazy number of files at…

a Wake-on-lan event is a special type of packet (frequently called a magic packet) that gets sent that triggers the computer to turn on (think of it as a software power button) I've got a roku but since my Amahi/Plex server is always on I've never investigated having the roku send a "Magic Packet" to wake up the…

you certainly can use a laptop as a server for some things, (file server, etc certainly) things that require lots of processing power I would avoid due to the fear of causing the laptop to over-heat.

Just for clarification Amahi 5 & 7 run on Fedora, Amahi 4 & 6 ran on Ubuntu.

Amahi can be set to wake-on-lan which I believe is what you're looking for, but then you'd lose using it as a DHCP, and DNS server along with a bunch of other functunality of it.

if all you're doing is setting up a VPN and file server that old P4 should be fine, might throw some more ram at it. Basically the Amahi requirements are whatever the underlying linux (Fedora or Ubuntu depending on your choice) system requirements are.

to expand a bit on Alan's reply (I've been using Amahi w/ Greyhole for about 4 years.) Basically Greyhole lets you choose which shares are duplicated on multiple disks to give you the best choice between free disk space and redundancy.

I've been using Amahi for about 4 years and love it.

have you tried hitting the windows key? about 50% of the time that will pop the start menu when in full screen games for me and I can open something then alt+tab and kill the game.