How to simplify macros - excel

Question

I'm new here and to the world of VBA.
I've been asked to create a spreadsheet which a user can enter a task number of a job they have been given, from this auto populate other columns with data from a master list, and then when they change the status of the job the row changes colour.
I know that this is easy to do using the vlookup function as a formula in each cell and conditional formatting.
I have searched the web and have found differnt macros that can do the job, but some of them take far to long to find a match, the master task list can contain upto 3000 items.
So I've very crudely cobbled together something that works and quickly.
I was wondering how I can clean these macros up as I'm sure the way I've done it will cause problems in the future or if there is a better way to do the job.
Also as a secoundry question is there any way to get a message box to pop up if the number the user enters doesn't match anything on the master list.
Private Sub Worksheet_Change(ByVal Target As Range)
Dim KeyCells As Range
Dim Status As Range
Set KeyCells = Range("B1:B700") 'Range Looking for Change
Set Status = Range("K1:K700")
Application.ScreenUpdating = False
If Not Application.Intersect(KeyCells, Range(Target.Address)) _
Is Nothing Then
Dim SourceLastRow As Long
Dim OutputLastRow As Long
Dim sourceSheet As Worksheet
Dim outputSheet As Worksheet
Set sourceSheet = Worksheets("Task List")
Set outputSheet = Worksheets("2016")
'Determine last row of Task List
With sourceSheet
SourceLastRow = .Cells(.Rows.Count, "A").End(xlUp).Row
End With
With outputSheet
'Determine last row in col B
OutputLastRow = .Cells(.Rows.Count, "B").End(xlUp).Row
'Put formula in relevant cell
.Range("F2:F" & OutputLastRow).Formula = _
"=VLOOKUP(B2,'Task List'!$A$2:$D$" & SourceLastRow & ",2,0)"
End With
With outputSheet
OutputLastRow = .Cells(.Rows.Count, "B").End(xlUp).Row
.Range("H2:H" & OutputLastRow).Formula = _
"=VLOOKUP(B2,'Task List'!$A$2:$D$" & SourceLastRow & ",3,0)"
End With
With outputSheet
OutputLastRow = .Cells(.Rows.Count, "B").End(xlUp).Row
.Range("G2:G" & OutputLastRow).Formula = _
"=VLOOKUP(B2,'Task List'!$A$2:$D$" & SourceLastRow & ",4,0)"
End With
End If
If Not Application.Intersect(Status, Range(Target.Address)) _
Is Nothing Then
Dim n As Long
iLastRow = Cells(Rows.Count, "B").End(xlUp).Row
For n = iLastRow To 1 Step -1
If Cells(n, "K").Value = "Closed" Then
Rows(n).Interior.Color = RGB(57, 255, 20)
End If
If Cells(n, "K").Value = "Bond" Then
Rows(n).Interior.Color = RGB(249, 255, 1)
End If
If Cells(n, "K").Value = "Eng Bond" Then
Rows(n).Interior.Color = RGB(255, 102, 0)
End If
If Cells(n, "K").Value = "Fail" Then
Rows(n).Interior.Color = RGB(255, 1, 1)
End If
If Cells(n, "K").Value = "" Then
Rows(n).Interior.ColorIndex = 2
End If
Next n
End If
Application.ScreenUpdating = True
End Sub