Expression.Call and Count

I am looking for a way to do the following dynamically:

var q = context.Subscription
               .Include("Client")
               .Include("Invoices")
                Where(s=>s.Client.Invoices.Count(i=>i.InvoiceID == SomeInt) > 0);

I would like to dynamically build an expression for the left side:

Expression left = s => s.Client.Invoices.Count(i => i.InvoiceID == iSomeVar); //!
Expression right = Expression.Constant(0);
var binary = Expression.GreaterThan(left, right);

Thank!

UPDATED NOTES:

Please note: the end result should be

Expression<Func<T, bool>>

Simple version:

// To give clear idea, all what I want to achieve is to determine 
// whether specific record exists in reference table using known Path. 
// Ultimately I want to extend following function (which works great by 
// the way, but for simple operations)

static Expression CreateExpression<T>(string propertyPath, 
                                      object propertyValue, 
                                      ParameterExpression parameterExpression)
{
     PropertyInfo property = typeof(T).GetProperty(propertyName);
     MemberExpression left = Expression.Property(parameterExpression, property);
     ConstantExpression right = Expression.Constant(0);
     BinaryExpression binary = Expression.GreaterThan(left, right);

     return binary;
}

// And I want to call this function and get result exactly as shown below:

Expression result = 
           CreateExpression<Subscription>("Client.Invoices.InvoiceID", 
                                          theID,
                                          valueSelector.Parameters.Single());

// Where result will be: 
//       t => t.Client.Invoices.Count(i => i.InvoiceID == theID) > 0;

Extended version:

// 1) I'm using Silverlight 4, EF, RIA.

// 2) At the server side I have a function GetSubscriptionsByCriteria
//   that looks about it:

public IQueryable<Subscription> GetSubscriptionsByCriteria(...)
{
      var query = this.ObjectContext.Subscriptions.Include("Client")
                                                  .Include("Client.Invoices");
      var criteria = BuildCriteria(...);
      return query.Where(criteria)
}

// 3) BuildCriteria(...) function gathers Expressions and 
//    aggregates it into the single Expression with different 
//    AND/OR conditions, something like that:

public Expression<Func<Subscription, bool>> BuildCriteria(
                      List<SearchFilter> filters,
                      Expression<Func<Subscription, bool>> valueSelector)
{
    List<Expression> filterExpressions = new List<Expression>();
    ...
    Expression expr = CreateExpression<Subscription>(
                                   sfItem.DBPropertyName, 
                                   sfItem.DBPropertyValue, 
                                   paramExpression, 
                                   sf.SearchCondition);
    filterExpressions.Add(expr);
    ...

    var filterBody = 
        filterExpressions.Aggregate<Expression>(
                (accumulate, equal) => Expression.And(accumulate, equal));
   return Expression
           .Lambda<Func<Subscription, bool>>(filterBody, paramExpression);
}

// 4) Here is the simplified version of CreateExpression function:

 static Expression CreateExpression<T>(string propertyName, 
                                       object propertyValue, 
                                       ParameterExpression paramExpression)
 {
        PropertyInfo property = typeof(T).GetProperty(propertyName);
        ConstantExpression right = Expression.Constant(0);
        MemberExpression left = Expression.Property(paramExpression, property);

        return binary = Expression.Equals(left, right);
 }

So, I hope it is now clear why I need an expression for the left side in my original post. Try to make it as dry as possible.

PS Not to make this too confusing, that’s why I think I need to do Expression.Call (...) : When I run the following code and break it to see DebugView, I notice this:

Expression<Func<Subscription, bool>> predicate = 
           t => t.Client.Invoices.Count(i => i.InvoiceID == 5) > 0;
BinaryExpression eq = (BinaryExpression)predicate.Body;
var left = eq.Left; // <-- See DEBUG VIEW
var right = eq.Right;   

// DEBUG VIEW:
// Arguments: Count = 2
//            [0] = {t.Client.Invoices}
//            [1] = {i => (i.InvoiceID == 5)}
// DebugView: ".Call System.Linq.Enumerable.Count(
//               ($t.Client).ClientInvoices,
//               .Lambda#Lambda1<System.Func`2[SLApp.Web.Invoice,System.Boolean]>)
//               .Lambda#Lambda1<System.Func`2[SLApp.Web.Invoice,System.Boolean]>
//               (SLApp.Web.ClientInvoice $i){ $i.ClientInvoiceID == 5 }"