Shellfish: Majority of scholars say this is allowed.
Seafood: All seafood is permitted (with the exception of shark according to a few scholars)
Kosher meat: Yes, this is permitted for Muslims, provided that you trust the source. If the meat has been kosher certified then this is OK for us; kosher certification is very strict and therefor we can trust the major certifications (K, U, D). If you're buying kosher products, watch out for wine as an ingredient, because Jews can have it in their food, whereas Muslims can't.
If you have access to halal meat, however, it is better for you to buy it over kosher meat.
However, don't let dietary restrictions stop you from becoming a Muslim. It can be hard at first, but know that the sacrifices you make in this life are all investments in Paradise.