I think 18 or 22 gauge wire is a little small for a wire antenna. Its a mechanical thing, not electrical. The size of wire makes little difference as far as the electrical part of this stuff. The important part is that there can be a lot of stress on it, it may break. Something like 14 gauge will typically handle the strain. Its your wallet, get any size wire you want (or what you happen to have already).
This particular antenna is advertised as not needing a match box or tuner. I think having one is still a good idea. (And no,tuners are not very lossy if adjusted correctly). There will always be some variation in input impedance, no two antennas are ever exactly alike, which is normal.