Variables In Dynamic SQL In A Stored Procedure

I am taking my first steps into stored procedures and I am working on a solution for efficiently paging large resultsets with SQL Server 2000 based on the example on 4Guys: http://www.4guysfromrolla.com/webtech/042606-1.shtml

The problem with my stored procedure is, is that it doesn't seem to recognize a variable (@First_Id) in my dynamic Sql. With this particular sproc I get the error message: "Must declare the scalar variable '@First_Id'"
It seems to be a problem with 'scope', though I still can't yet figure out. Can anyone give me some hints on how to correctly implement the @First_Id in my stored procedure? Thanks in advance!

Here's the sproc:

ALTER PROCEDURE dbo.spSearchNieuws
(
@SearchQuery NVARCHAR(100) = NULL,
@CategorieId INT = NULL,
@StartRowIndex INT,
        @MaximumRows INT,
@Debug BIT = 0
)
AS

SET NOCOUNT ON

DECLARE @Sql_sri   NVARCHAR(4000),
@Sql_mr    NVARCHAR(4000),
@Paramlist NVARCHAR(4000),
@First_Id  INT,
@StartRow  INT


SET ROWCOUNT @StartRowIndex
SELECT @Sql_sri = '
SELECT @First_Id = dbo.tblNieuws.NieuwsId FROM dbo.tblNieuws
WHERE 1 = 1'

IF @SearchQuery IS NOT NULL
SELECT @Sql_sri = @Sql_sri + ' AND FREETEXT(dbo.tblNieuws.Nieuwskop, @xSearchQuery)'
    
        IF @CategorieId IS NOT NULL
SELECT @Sql_sri = @Sql_sri + ' AND dbo.tblNieuws.CategorieId = @xCategorieId'

SELECT @Sql_sri = @Sql_sri + ' ORDER BY dbo.tblNieuws.NieuwsId DESC'



SET ROWCOUNT @MaximumRows
SELECT @Sql_mr = '
SELECT dbo.tblNieuws.NieuwsId, dbo.tblNieuws.NieuwsKop, dbo.tblNieuws.NieuwsLink, dbo.tblNieuws.NieuwsOmschrijving, dbo.tblNieuws.NieuwsDatum,
                dbo.tblNieuws.NieuwsTijd, dbo.tblNieuws.BronId, dbo.tblNieuws.CategorieId, dbo.tblBronnen.BronNaam, dbo.tblBronnen.BronLink, dbo.tblBronnen.BiBu, dbo.tblBronnen.Video,
                dbo.tblCategorieen.Categorie
FROM       dbo.tblNieuws INNER JOIN
                dbo.tblBronnen ON dbo.tblNieuws.BronId = dbo.tblBronnen.BronId INNER JOIN
                dbo.tblCategorieen ON dbo.tblNieuws.CategorieId = dbo.tblCategorieen.CategorieId AND
                dbo.tblBronnen.CategorieId = dbo.tblCategorieen.CategorieId
         WHERE dbo.tblNieuws.NieuwsId <= @First_Id
         AND 1 = 1'
         
     IF @SearchQuery IS NOT NULL
SELECT @Sql_mr = @Sql_mr + ' AND FREETEXT(dbo.tblNieuws.Nieuwskop, @xSearchQuery)'
    
     IF @CategorieId IS NOT NULL
SELECT @Sql_mr = @Sql_mr + ' AND dbo.tblNieuws.CategorieId = @xCategorieId'

     SELECT @Sql_mr = @Sql_mr + ' ORDER BY dbo.tblNieuws.NieuwsId DESC'

IF @Debug = 1
PRINT @Sql_mr 

SELECT @Paramlist = '@xSearchQuery NVARCHAR(100),
     @xCategorieId INT'

EXEC sp_executesql   @Sql_sri, @Paramlist,
     @SearchQuery, @CategorieId

EXEC sp_executesql   @Sql_mr, @Paramlist,
     @SearchQuery, @CategorieId