Migrating from MS SQL server

Hello,

I'm just new on this list and hope that I don't ask a FAQ (if so please
lead me to the correct document).

I have to migrate a smaller Database (about 20 tables, fully normalized,
no table more than 1000 lines) for a web-project from MS SQL server
to PostgreSQL.

Each table has an unique ID which I plan to port to a SEQUENCE.
Because there were some deleted rows the current database I'm affraid
that the PostgreSQL tables will not be filled correctly because
the IDs are refered to in other tables and are not possible to
change.  I wonder how to assure the referential integrity of the
tables.
Are there any mechanisms in PostgreSQl to assure that only those
numbers could be included into a table which are present as ID in
a certain row and how to do that if it is possible?

Furthermore I have some stored procedures.  Do I have to expect
many problems when converting to PostgreSQL?

If you answer my questions please keep in mind that I consider
myself as a beginner in SQL and databases and as quite experienced in
handling Linux.

Kind regards

           Andreas.