As I posted in the Alpha vs Hahn turbo thread, there haven't been a whole lot of Slingshots converted to automatic transmission, so they are a pretty rare breed as far as resale concerned.
The only firms now offering auto conversion (that I'm aware of) are DDM Works (in SC) (also does turbos and SCs), Bullet Speed & Design (FL), Cycle Springs (installs the Bullet Speed conversion and may still do boosted setups, too) and Performance Autowerks (IL) (auto trans and SC/TC installs). They may also have relationships with other firms around the nation that can do the conversion, so your best bet os to contact each company for info. You might even ask them about mods to accommodate your specific needs.
Converting to an auto trans, not to mention adding boost, will pretty much cancel any warranty coverage since so many systems might be impacted, so buying a used Slingshot to mod may be your best bet. A typical turbo install will probably run between $5-8K and an auto trans conversion will probably also run about the same. Finding a used turbo-equipped Slingshot should save some money as most accessories don't return more than about half their cost max. Finding a good used Slingshot will probably cost around $15-22K depending on year/model and accessories plus the cost of whatever you still need added. With luck and patience, you should find a decent turbo'd Slingshot for around $18-22K and then figure another $10-12K or so for the automatic trans conversion and access mods such as a lift, removable steering wheel, etc.
Good Luck!