The TP-Link TL-WR703N is the smaller brother (packaging-wise) of the TL-MR3020, which some of you know as as the RIPE Atlas Probe V3. As far as I know, the TL-WR703N is made for the Chinese market (which explains the heavily encrypted Web interface it comes with :-), but it's easily obtainable in Europe, via Amazon for example.
The package costs EUR 25.00 and includes the router, a charger (with a Euro adapter), a USB cable, and an instruction booklet in Chinese. It's less capable CPU and memory-wise than a Raspberry Pi, but it has WiFi on-board and it's nicely packaged. The following image shows the footprints of both devices: the larger one is a Raspi board without a case, the smaller one is the TL-WR703N:
Both devices are ideally suited to run a DNS server on them (e.g. Unbound) or an MQTT broker (e.g. Mosquitto). They won't win any prizes for performance, but they sustain around 1500 MQTT publishes per second, which is more than enough for a small installation. Frederic Cambus is running the TL-MR3020 with Unbound and Nginx on it, and seems to be happy with it. :-)
I added a small USB memory key (that's the "wart" you see sticking out of the device) and configured the system to boot from that with what is called ExtRoot, and the result is a TL-WR703N with space galore onto which I can install packages until the cows come home.
opkg update opkg install ...
If you just want to add external storage (without having the router boot from it) you can do so very easily.