The counter man at an auto parts store should be able to steer you in the right direction for the proper gasket material. That is if he's worth his salt.
As for silicone gasket sealers. If you read the labels, most of them will say, "Not compatible with gasoline".
If the right gasket material is used you shouldn't need an additional sealer. Just torque the fasteners correctly, retorque after the engine has reached operatintg temp a couple of times then forget about them.
Thickness: Exhaust, I like a minimum of 1/8th inch. The intake manifold to cylinder can get by with much thinner gasket material. As long as it is gasket material...not cereal boxes or envelopes.
The intake and exhaust gaskets that come with most kits are questionable at best. I replace those but as for the head and base (cylinder to case) gasket...just make sure they're torqued correctly. There is no need to replace them on a new engine.
Tom
