The Anne Pro 2 is a solid wireless choice at a great ($80) price point. You can find it in various MX, Gateron, or Kailh versions. The case is plastic, but you could probably find a metal case that would be compatible (it just needs to have room for the battery). As for keycaps, you can easily find a blank set for it.
If money is no object, consider the Leopold FC660C or the HHKB. Both have plastic cases, but you can find really nice metal cases for the Leopold if your cash flow allows. The HHKB comes in both wired and wireless versions as well as printed or unprinted keys. The Topre keyfeel is leaps and bounds above MX brown: really tactile and just amazing to type on. The Leopold and HHKB wired both come in silenced versions for a bit extra cash, but I would recommend getting the regular versions unless you are worried about noise. You can always install silencing rings if you don't mind opening the case.
Lastly, kbdfans.cn has several options for 60% prebuilt keyboards with metal cases.