Data Model Design For Query Performance

I have an opportunity to rebuild a database model with the express purpose of improving query performance. So given the following I have a few questions.

Table A (~500M records)
Primary Key Field (int)
Field 1 (varchar)
Field 2 (varchar)
Field 3 (varchar)
Field 4 (varchar)
Field 5 (varchar)

Table B (1B+ records)
Primary Key Field (int)
Foreign Key Field (int)
Field 1 (varchar)
Field 2 (varchar)
Field 3 (varchar)
Field 4 (varchar)
Field 5 (varchar)

* Assumed: Tables are inner joined on all queries. The database is readonly.

-- Most of my lookups are based on querying Field 1 of Table A. The data content of Field 1, Table A is 90% unique.
1) Would it be more beneficial to put the clustered index on Field 1 instead of the PK field in Table A?
2) Can an Identity column be non-clustered?
3) Alternatively, would it be beneficial to build a separate lookup table with just the PK & Field 1 of Table A, with a clustered index on the lookup table Field 1 which I join on Table A? (did that make sense?)

-- I have a secondary lookup that performs queries on Fields 1, 2, 3, 4 & 5 of Table B
1) Would it be more beneficial to create an additional indexed lookup column of the concantenated values of Fields 1-5 of Table B versus a covering index of all 5 columns?
2) Does a clustered index have to be unique?
3) Would a clustered index be more beneficial over Fields 1-5 or the special lookup column versus the PK or FK fields?
4) Would creating a special lookup table with just the requisite fields be more beneficial?

An extra question. The existing data model uses the CHAR datatype for all columns less than 9 characters wide and the columns are set to allow nulls. This requires every select statement to COALESCE() and RTRIM() all these columns. I intend to make all (affected) columns VARCHAR, NOT NULL with a default value of a 0-length string.
Will this enhance query performance?

Thanks in advance for any insight.