I'm using pfSense myself - and plan to do something similar at some point VLAN wise if required, aside for that I'm using roughly what you're looking at.
1. How about a Netgear GS108E (web managed, desktop, fanless, 8 port) which seems to be around £30 ish. Lifetime warranty, too..
2. I recently replaced a very old single core Atom box I've had running for years with an Intel Apollo Lake Celeron (a Gigabyte J3455N-D3H motherboard in my case) which has two gigabit LAN ports. This is in a very small mini ITX case along with a picoPSU, a 4GB stick of DDR3L, and a 16GB SSD. Then a decent 60W 12V laptop style PSU. This solution has been fast and rock solid so far (3 months and counting at this point), and supports AES-NI to accelerate VPN usage. It's passively cooled with zero moving parts. Dead silent, plenty of performance. Not sure how much power it's using but I'm sure it's not much! Not totally certain on cost but I think under £200 for sure. Self build.
3. I use Ubiquiti points - Two AC-Lite APs in my case. I've got them cabled at opposite ends of the house in my case, the range seems pretty good (tested far end of house using a single point plugged in) but I'm not certain you'll get an amazing signal through multiple walls and then at a distance in the garden beyond. Perhaps an AC LR might be more suitable? Not sure really - only way to tell would be to try it!
Consider that to use these AP's (they have no web interface in their own right) you need to run the controller software, buy a 'cloud key', or use a cloud-based controller to manage them. Personally I run mine on a VM on the Google Compute Platform.
If you REALLY can't get an AP closer to the garden, you could buy a second AP and have it uplink wirelessly to the first using the Unifi controller software. That way it can rebroadcast the signal much closer to the garden. Instructions to do this is HERE
Just bear in mind the potential performance losses using a wireless uplink, given it's having to relay wirelessly.
Edited by summat (Tue 08-Aug-17 20:47:56)