No special seats required. One of the Squirrels features is that all the seats come out relatively easily for configuration changes. You sit on the structure in a Bell 206 (series)
To be certified, you would need a “stretcher kit” and matching stretcher.
There are many options – all ridiculously priced, I’m sure.
Probably plenty for sale SH, as the world moves on from EMS Squirrels.
The two LH rear seats are usually removed – sometimes replaced with very short seats for the Medic/DR to sit at the patients head.
I believe one of the Island Hopper kits was manufactured and “certified” locally.
I flew many Medevacs in Fiji – Latoka Hospital (House of Death). Suava “Don’t land on the roof, because ‘something’ might happen” Hospital.
Good luck.
Watch the rapidly changing weather. There is plenty of evidence to suggest that some rain is too heavy to fly through.