The Haswell CPUs, which use the 1150 socket, are what you are working with.
So, then, you'll be looking for motherboards that support Haswell CPUs. Manufacturers, such as Asus/MSI/Gigabyte, all offered a range of motherboards from inexpensive to high end. The chipsets were H81 on the low end to Q85/87 in the mid end to Z87 on the high end.
Since you mentioned mid range, I'd suggest looking at the Q85 or 87s on the used market. They should include 8 PCI-ex 2.0 expansion slots and 4 memory slots, max memory supported would depend on the make and model.