Reddit mentions of SanDisk Cruzer Fit 16 GB X2 = 32GB USB Flash Drive SDCZ33-016G-B35-2PK W/Everything But Stromboli (Tm) Lanyard

mini-itx

2.0Ghz, Latest Goldmont AES-NI

VT-X&D

ebay i340-t4

PCIe v2.0 x2 mode = 8.0Gbits/s bidirectional for 4x gigbit NICs

8GB DDR3L 1866 SODIMM

2xSATA

No moving parts at all if you go pico-psu

https://www.newegg.com/Product/Product.aspx?Item=N82E16813157726

http://www.ebay.com/itm/IBM-Intel-I340-T4-Ethernet-Quad-Port-Adapter-94Y5167-49Y4241-/172489524601?hash=item28292d4579:g:1XgAAOSwt5hYfV9W

https://www.newegg.com/Product/Product.aspx?Item=N82E16820233581&cm_re=ddr3l-_-20-233-581-_-Product

http://www.ebay.com/itm/PicoPSU-80-12V-DC-DC-ATX-Power-Supply-w-60W-AC-Adapter-/191942916682?hash=item2cb0b07a4a:g:hqsAAOSw0UdXrdzB

or

https://www.newegg.com/Product/Product.aspx?Item=9SIA24G3RH5343

https://smile.amazon.com/Silverstone-Computer-Front-Panel-SG13B/dp/B00U8IS89E/ref=sr_1_13?s=pc&ie=UTF8&qid=1486056019&sr=1-13&keywords=mini+itx

https://smile.amazon.com/Transcend-Half-Slim-Solid-State-TS16GHSD370/dp/B00OPHX8XK/ref=sr_1_11?s=pc&ie=UTF8&qid=1486056413&sr=1-11&refinements=p_n_feature_three_browse-bin%3A14027456011%2Cp_n_feature_keywords_two_browse-bin%3A4929543011

or with 2.4 coming out your could mirror a pair of USB's in ZFS

https://smile.amazon.com/SanDisk-SDCZ33-016G-B35-2PK-Everything-Stromboli-Lanyard/dp/B00DTZA5S0/ref=sr_1_8?ie=UTF8&qid=1486056220&sr=8-8&keywords=sandisk+cruzer+fit

Whole system, assuming you don't have any parts laying around you can reuse: $222 - $252 depending on whether you want pico-PSU v PSU and USBx2 v SSD.

Assuming you need wifi since it's a bday gift, either use existing wifi routers as AP's or get some ubiquiti's.
https://www.amazon.com/Ubiquiti-UAP-3-802-11n-Wireless-Access/dp/B00HXT8RDI/ref=pd_sim_147_1?_encoding=UTF8&psc=1&refRID=TH28JC9DA8W3MRNCQQD0

I'm pretty new to it too, and for me, buying the kit (power cable, hdmi, preloaded microSD, etc.) was a good route. Then you have a choice to make - is it a tiny linux server for you, or do you want to play with making stuff? I am pretty new to it all, but the Raspberry Pi T-cobbler + ribbon cable + breadboard kit (transistors, leds, wires, etc.) was the right combo for me to start learning. From another newbie, the best advice i can give is to watch your voltages!

My Pi: https://www.amazon.com/gp/product/B00L87YMGM/ref=oh_aui_detailpage_o06_s00?ie=UTF8&psc=1

The add-ons:
https://www.amazon.com/gp/product/B00N0R6EVE/ref=oh_aui_detailpage_o07_s00?ie=UTF8&psc=1
https://www.amazon.com/gp/product/B009NC8OS6/ref=oh_aui_detailpage_o08_s00?ie=UTF8&psc=1
https://www.amazon.com/gp/product/B00N4SHO5O/ref=oh_aui_detailpage_o08_s01?ie=UTF8&psc=1
https://www.amazon.com/gp/product/B00D9MK150/ref=oh_aui_detailpage_o08_s01?ie=UTF8&psc=1

More storage:
https://www.amazon.com/gp/product/B00DTZA5S0/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1

With all of that, plus an outlet/box/misc wire from the hardware store I'm now able to control an electrical outlet remotely - pretty basic, but a huge deal for me.

What is the 250gb SSD for... Caching? If it's your boots drive that's completely overkill. FreeNas is known for booting from a USB. Better yet, buy two of the same USBs and setup a redundant boot drive in case one fails. Here's a good Amazon deal: SanDisk Cruzer Fit 16 GB x2