From procedural language to Power query

Is this what you mean?  It yields your desired results but not sure if it's in the way you are hoping for (but it does make a record dependent on the p1 value).  To see how it works, just create a blank query, go to Advanced Editor, and replace the text there with the M code below.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUaoAYnOlWJ1oJSMgqxKILcA8kFwVEJsqxcYCAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [p1 = _t, p2 = _t, p3 = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"p1", Int64.Type}, {"p2", type text}, {"p3", Int64.Type}}),
    #"Added Custom" = Table.AddColumn(#"Changed Type", "Custom", each if [p1] <= 1 then [newP2 = "a", newP3 = [p3] + 1] else [newP2 = "b", newP3 = [p3] + 5]),
    #"Expanded Custom" = Table.ExpandRecordColumn(#"Added Custom", "Custom", {"newP2", "newP3"}, {"newP2", "newP3"}),
    #"Removed Columns" = Table.RemoveColumns(#"Expanded Custom",{"p2", "p3"}),
    #"Renamed Columns" = Table.RenameColumns(#"Removed Columns",{{"newP2", "p2"}, {"newP3", "p3"}}),
    #"Changed Type1" = Table.TransformColumnTypes(#"Renamed Columns",{{"p2", type text}, {"p3", Int64.Type}})
in
    #"Changed Type1"

Regards,

Pat