Both the CB and CBi are available as three seaters... there is a fairly straight forward seat conversion.
Mind you... 3 up in a C was bad enough... youd only want to weigh about 65KG to get anywhere.
Great little machines
I could be wildly wrong but Im sure the following is near true
C-Lycoming H10-360 D1A 190HP injected 2050 lbs MAUW
CB - Cant remember but I think 180HP Carb 1750lbs MAUW
CBi - HIO-360-G1A Engine 180HP injected 1750lbs MAUW
As far as times and stuff... you will find the Schweizer website has a full TBO list for as parts on both available types (they dont sell CB's anymore)
http://www.sacusa.com/helicopters/300c_overhaul.asp
http://www.sacusa.com/helicopters/300cbi_overhaul.asp
There are also standard equipment lists for each type at the site... have another look
http://www.sacusa.com/helicopters/300cbi_equipment.asp
http://www.sacusa.com/helicopters/300c_equipment.asp