Love the case!
You sound like me... I know what you mean by you don't really have a budget to get the job done.
I am not "THE" expert when it comes to builds, but I will share what little knowledge I have. I am sure others will chime in. I am using a Freeware OS for my NAS and I also have XBMC on my HTPC and other hardware. I currently experience no buffering or artifacts while streaming over wi-fi. I am not sure if your hardware will all be directly/wirelessly connected to the network or a combination of both? I will assume the latter. You may need the use of wireless repeaters if you have a larger than average home for wi-fi. Can I ask what is the average size of your movie media? I successfully tested my wi-fi for 30-35GB HD ISO files with no buffering. However I compressed my final BD media files down to 7-8GB to save HD space.
Below are the equipment I tested over wi-fi successfully.
Up to a week ago, COX was my provider. I used the following hardware.
MODEM:...........I bought Motorola Modem -SURFBoard Model SB6121 --called cox and had it configured, since it was compatible with their systems.
ROUTER..........I use the D-Link DGL-4500 Extreme-N Selectable Dual-Band Gaming Router to connect to the modem
With Fios I currently have:
MODEM............Supplied Verizon Fios Modem with built-in router and wi-fi
ROUTER..........I still use the D-Link DGL-4500 Extreme-N Selectable Dual-Band Gaming Router to connect to the Fios modem
All my PCs/laptops are using the default client for wi-fi or network cards built-in to the respective Mobo's. I am not using any external PC-Card adapters or any additional separate third party internal network cards. The NAS is connected to the Dlink modem directly, as well as, my Main Full tower PC. All other hardware are connected wirelessly to the Dlink router (wireless network printer, TV's, BD-Player, laptops, HTPC and other devices). However, I still have the available option to connect to the Fios wi-fi if needed. My NAS build is a bit over the top so you may not want to follow that build exactly.
Here are links to my HTPC and NAS build. An excel sheet with the items used are posted on those links below, as well as, a reason for my choices.
Hope my information helps a tad.