A 1 in 18" twist would be wonderful for keeping powder charges tiny and light without losing accuracy with roundballs, along with providing for use with mild to moderate loads for conicals. Sounds like the perfect combination rifle for small game, target, and even large game; but I am not aware of any manufactured barrels, or even complete rifles with that fast of a rate of twist. In fact, the fastest rate of twist on a rifle that I can think of off hand was exhibited by the tc cherokee, coming in with a twist half as slow as your specifics, (and their fast twist had the market cornered on accuracy with light squirrel loads for years)...
There may be 1:18" rifle barrels out there, but my guess is that you will struggle finding many barrels with a twist that does more than one turn in 48". Typically, it is pistol barrels that exhibit these faster twists in the 1:20"s. And this is due to necessity as the incomplete burning of the powder in shorter barrels lightens the effect of a given powder load requiring greater torque (provided by increasing the rate of twist) to start that ball spinning due to its lesser velocity. That is also why longer barreled rifles give greater velocity AND get by with much slower rates of twist than do pistols and short rifles. Anything from 1:48 to 1:66 is more than adequate for roundball accuracy, although your loads for conicals at 1:66" would need to be incredibly hot (I would choose the 1:48 if you intend to use conicals often).
You really should be fine with anything greater than 1:48 if using roundballs. Any twist faster would require loads that are much lighter, and also leave you with a much steeper arc of trajectory... which, as a target rifle, that would be very undesirable since target and match rifles need to shoot flat...in addition, you would never be able to use hot loads accurately in the rifle as they would strip lead straight from the rifling's grip before any spin is imparted. Thats a lot of turns (2 of them) to accelerate a ball through over a 36" run. A hot load in that barrel would have an effect on your roundball similar to mashing the accelerater on a corvette has on its wheels: that is, torque and lack of opposing traction (a burnout).