Presuming you want to keep the battery in the rear and not the front you'll need to source a battery stock tube. The standard one doesn't have any space to feed the wires into the tube properly and is only meant for front wired guns. A battery stock tube has a channel in the bottom for the wires to run in. You will also need something like a Crane stock as well if you want to use anything other than small Lipos as your power source.
As regards a metal receiver set, it depends a lot on how much you want to pay and whether you get a full set, or just the bare receivers without the pins, ejector cover, cocking handle, etc, although you should have those from your existing set anyway. You can pay anything between about £40 and £150 for a metal receiver. Redbull UK do some nice Madbull Daniel Defence ones for about £95, or you can get Hurricane full kits from Airsoft World for between £125 & £150.
