Expression Too Complex Access 2007, how can I simplify this IIF sequence?

Question 1

Here is how I would approach it:

I'd create a saved query in Access named [CountPreviousSalesOrderLines]

PARAMETERS prmID Long, prmSalesOrderNo Text(255);
SELECT COUNT(*) AS n
FROM [Peachtree-Import-Dist]
WHERE SalesOrderNo=[prmSalesOrderNo] AND ID<=[prmID];

Then I would add a standard Module to the database that included the following function

Option Compare Database
Option Explicit

Public Function getSODist(ID As Long, SalesOrderNo As String) As Long
    Dim cdb As DAO.Database, qdf As DAO.QueryDef, rst As DAO.Recordset

    Set cdb = CurrentDb
    Set qdf = cdb.QueryDefs("CountPreviousSalesOrderLines")
    qdf!prmID = ID
    qdf!prmSalesOrderNo = SalesOrderNo
    Set rst = qdf.OpenRecordset(dbOpenSnapshot)
    getSODist = rst!n
    rst.Close
    Set rst = Nothing
    Set qdf = Nothing
    Set cdb = Nothing
End Function

For the sample data in [Peachtree-Import-Dist]

ID  SalesOrderNo
--  ------------
 1  001         
 2  001         
 3  001         
 4  001         
 5  002         
 6  003         
 7  003         
 8  003         
 9  002

the query

SELECT ID, SalesOrderNo, getSODist(ID,SalesOrderNo) AS SODist
FROM [Peachtree-Import-Dist]

returns

ID  SalesOrderNo  SODist
--  ------------  ------
 1  001                1
 2  001                2
 3  001                3
 4  001                4
 5  002                1
 6  003                1
 7  003                2
 8  003                3
 9  002                2

For best performance, make sure that the [SalesOrderNo] field is indexed: Yes (Duplicates OK).

Question 2

It looks like you are trying to get a running count of each [SalesOrderNo]. is that right?

In a new module, paste the following code:

Function RunningCount(WhatToCount As String) As Integer
Static CountSoFar As Long, var  As String

If var <> WhatToCount Then       '  Is WhatToCount different from the one before?
        CountSoFar = 0
        var = WhatToCount        ' save the value of this one for comparison with the next
End If
    CountSoFar = CountSoFar + 1  ' increment the variable
    RunningCount = CountSoFar    ' return the result to the query
End Function

In the query, you use the function like this:

SODist:RunningCount([SalesOrderNo])

Make sure the query sorts on [SalesOrderNo]