Steve,

What you want is an HT.

I sail my HT singlehanded and its great, the only issue is that over 15knots you are over powered. I've toyed with getting a smaller main for the HT, but if its windy and I want to sail single I take out the Acat.

As far as an Acat for double handed, I wouldnt recommend it, everything on an Acat is built for one person and adding another full size adult is going to break stuff.

I would check out the HT in florida for $4k and then spend a couple bucks lightening it up with Carbon beams (which I would widen to 8'6") for $800 - Forte RTS,

And get a reefable sail.

Everything else is going to be a camel.

And by no means will you be able to keep an HT mast under an Acat, that will be the twitchy-est boat alive, dont do it, it will pitchpole and bury the bows at the slightest puff.

The DK as a double hander isnt something I would recommend either, its designed as an Acat, not a double hander.

I cant comment on the M18, but since they dont really exist in the US, its not an issue.

If you get some marstrom Acat hulls, pay a naval architect to work out the rig, get carbon beams from Forte, and a Hall mast to fit. Custom main. Except now you have a one off design and wont really be able to race it.

Save yourself the headache and get that HT. If you are up by me I've got one you check out.
Bill