Have at it, but I think even the monopole 0 cost antennas would work at that range.
A 1W-ish 5V panel sounds good with the 7.5F supercap. I haven't gone through a winter (short days with lots of overcast) yet, but i think outside a 1W panel should charge that cap even with not lots/great sunlight - maybe not snow/ice on it - although wouldnt that be kinda like my glass blocks?
If your charging the supercap enough, full power should be good enough. If the cap can't handle it, add another in parallel.
You can put an SMA (with a little care) on the small regular Moteinos. In R6 I enlarged the ANT pad on both sides and added extra GND pads to help solder an SMA on it. There's even a dedicated u.FL pad on TOP. There's SMA pigtails on ebay, they're SMA case mount on one side, and plain coax on the other, so you can just solder the shielding to GND and feed wire to the ANT port, just trim the coax length to 1/4 wavelength multiples.