I used to sail my Inter 20 solo quite often but on a lake. I flipped it a few times gybing in big wind but had no trouble righting it as long as there was enough wind to flip me, there was enough to right it, I weighed about 190. But I was on a lake, not out in the big ocean so I didn't have to contend with big waves.

The boat is easily controlled single handed and with those long, big fat bows, it is much more difficult to pitch-pole when going downwind than any other. I tried sailing it without the jib but I didn't change my mast rake and there was too much weather helm. I liked it with the jib better, even in big wind, as you can ease the traveler out, pull the boards up and ease the main sheet in the big puffs, and just cruise along with the jib working, I found that was easier than not using the jib.

If you sail the F18 solo, the Inter 20 won't be much different, and as long as you get the wind under the mast you can right it solo, but I think you will find it more foregiving and less likely to flip going downwind.

Oh, you will need double Harken auto-ratchet blocks for the spinnaker in any good wind, to keep from ripping your arm off.