Using id ranges to distinguish customers from suppliers is a bad idea. Don't.
The problem is in the domain of accounting/bookkeeping. The standard in that domain is to have two binders, one for the invoices from suppliers and one for invoices to customers. That is one reason for having two tables.
Another reasons for having two tables is that in the order table there will be a field with the amount of the order/invoice. Is that going to be negative for supplier orders? Then what happens if you need to credit a customer for part of their order? Or you are credited part of a supplier order?
Two seperate tables does not get complex, at worst some duplication of logic. But do take care and see how you best can reuse code across the two tables, as there are similarities that can benefit from code reuse, but as you go along more differences will appear which are easier to handle with two tables.
With only one table each difference can lead to a lot of ifs or switches.
Two tables should not be slower in any way, but one table could lead to more complex queries that could be slower.