so basically my choices are:
~$600 = Current MTB + Xtracycle ($489) + rigid fork ($65) one bike solution

~$750 = Find-a-Bike ($250) + Xtracycle ($489) two bike solution, still have FS MTB...

~$1400 = Big Dummy Frame ($1050) + Longtail Kit ($279) one bike solution and keep old frame for what-if

Thanks everyone... I think I'm just going to convert my MTB, as seems to be the most straightforward thing to do... The Big Dummy Frame is nice, but I can always move up to that later and move all the parts over, or reconstitute my MTB then...