Just a heads up that running a PC only as a NAS gets costly. The electric bill on running a computer 24/7 that draws 100 watts in my area is 43 cents a day. I had a server that was for NAS and running my Database and I moved it onto an old single-core Sempron 3300+ laptop because the laptop drew 17.2 watts idle with display off with cover closed as measured by my Kill-A-Watt measuring device. The laptop costs me 8 cents a day to run it as a DB Server and NAS. I have better faster dual and quadcore laptops available, but those draw 45 watts to 65 watts continuous.
Now for the 10mbps issue it can be wrong driver used, or cable issue. I have seen network adapters with issues with a cable sync at 10mbps where 100mbps was unstable. Or if you have a switch or hub between them that is 10mbps it will force a bottleneck.
Wireless speeds depend on how good the signal is. For example I can get 54mbps if I have a strong signal on my wireless but if I only have 2 bars then it synch's at around a 12mbps.
If you can, I would go with CAT5e or CAT6 connected for all important and connections you want to keep secure in your home. Only go wireless if you have good quality signal of 3 bars or stronger, on a 5 bar scale, from locations that you need to use the devices in.
Lastly, a garage may be subject to humidity and weak temperature control which can destroy the computers components and circuitry.
Years ago I made a cabinet and had a PC located inside it and I wired up a home thermostat in it for a hot and cool cycle to maintain a constant 70 F inside the cabinet. As a heat source other than the PC itself which created heat I had a 100 watt light bulb which would heat the area inside the insulated cabinet to maintain pretty close to 70F even in the heart of winter with this cabinet out in my shed behind my mobile home. This was before cloud storage became available. When I got my Google Drive for 15GB of free offsite storage, I shutdown this cabinet and retired the old computer that was racking up an electric bill.
Humidity was still a problem though as I thought keeping a constant temperature inside would avoid that as an issue but the case and main board after 3 years of use were corroded inside with solder legs with white chalky lead oxide and the computer case had rust on the bare metal surface inside very lightly.
There are better solutions out there than running a PC as a NAS these days. A Desktop PC as a NAS really only makes sense if the system is doing more than NAS such as mine which runs a database so its multipurpose, yet I chose the low power demand of an older laptop with a low power consumption processor.
Years ago I was using FreeNAS for my NAS and not a Windows Share.
http://www.freenas.org/Because I am using my laptop as a database server and storage, and its isolated on a private network, and the laptop only has 512MB RAM I am using Windows XP SP3 to which there is only about 20MB of memory free, but it serves its purpose without any troubles. Not the fastest laptop out there, some scripts I run server side take a few minutes to complete, whereas on a modern computer it would be done in like 10 seconds or less, but I multitask so I tell it to run a script and then I do something else and come back to it when its completed so no wasted productivity.