If you need a NAS server with DLNA function, why not just get WD My Cloud for 2TB? It's around RM450ish and do the above function except downloading.

You need a functional PC for downloads and route the files into the server.

If you don't have problems with D-Link product, maybe you can consider DNS-320L http://www.dlink.com/uk/en/support/product...orage-enclosure.

If i remember correctly, it only cost me RM235 for DNS-320L and RM225 for WD RED 1TB. Quite reasonable price for me.
As price is way very cheap compared to Synology, so don't compare the app and software to Synology. This NAS-320L come with very basic app but still bundle with torrent downloader, ftp for file sharing, and CIFS/SAMBA/NFS for local file sharing.

And if you dare enough and have basic linux for simple editing and package installation, you can install other software to utilize it.

For my setup:

1. I share NAS drive to my PC for easier access. Also for my android. Can streaming movie.
2. Enable FTP to share movie to my friend through public / internet.
3. Install fun_plug to enable / easier to install package.
4. Install Transmission to replace stock torrent downloader (older version transmission), for torrent download.
5. Install Automatic for Transmission to automatically download TV series / Anime which come out weekly. This can be done by preconfigured rss for download link.

I previously did think of doing the same project as you with my dad as we wanted to setup the our home equipment to connect to a simple storage mainly for media sharing. But then we gave up the thought as the construction cost came up to be not worth the time and the power consumption would be higher (the idea of an NAS is to have it left on for long periods of time so power consumption is overall an important matter here).

You're looking at the Synology DS214SE which is quite a high end model when it comes to NAS. If you're building your own NAS the cost would definitely be cheaper but in the long run you would probably be better off buying a a factory manufactured NAS as the overall power consumption would be lower and the equipment are made specifically for handling storage and networking functions (ASIC, application specific integrated circuits) which would perform better than any NAS you could make at a low budget.

But I still applaud your for wanting to try this out. What I would suggest is:

1. Build your own rig : This is what you're planning but I would suggest you buy a new hardisk, the whole idea of a NAS is the storage so using a reused hardisk could prove to be a risky move, especially if it's going to be left on the whole day. You could get an enterprise grade disk or at least a WD Red, 2 pieces if you want redundancy, but doing this would put you back to square 1, the cost would come up to the same as an NAS.
2. Buy a router/switch with NAS function, some of these routers/switches do provide basic download applications as well (namely torrent apps) and work with USB connected hardisk, performance overall not impressive but it'll do since the whole idea is to keep it running 24/7 so this would save you some RM in the long run.
3. Buy a NAS

The only reason why you would actually build your own NAS would actually be for the ability to customize it and like you said, self satisfaction. But remember when there are more moving parts there is going to be higher chance of breakdowns as well and with a NAS what we aim for is reliability. Custom NAS would give you flexibility but reliability would depend on what you build it with, cut too much cost and your risk of losing your data would be higher, thats why factory made NAS are so expensive. I have a WD MyBook Live 2Tb and it works beautifully, plus it's portable since I carry it around when I travel.

One famous choice is get the HP N40L and install Xpenology.
Great performance and flexible, N40L is fully stable on Xpenology too.

There's a NAS thread on another section anyway - https://forum.lowyat.net/topic/2931469

Prebuild NAS are expensive eg.Synology but considering the fact that the software have been tested to be fully stable on the hardware compared to self build which you may face a few hiccups if it is not configured properly.

err...the board and processors are recycled items?
how many SATA does the board have?

just thinking that a lower power consumption CPU might be better?

wont the intel J1900 or those newer N3150 do a better job at a lower power consumption?
I would prefer one with at least 4 SATA port. wont know those PCIE compatibility with NAS software and sutff.
4 SATA port at least can run RAID 5 if needed.

hope you dont mind my 2 cents.
I am planning for a DIY NAS as well, just no budget for the HDD 

what's the individual costing for each of the items?

wont the intel J1900 or those newer N3150 do a better job at a lower power consumption?
I would prefer one with at least 4 SATA port. wont know those PCIE compatibility with NAS software and sutff.
4 SATA port at least can run RAID 5 if needed.