A C8 is just an M4. The differences are minor overall. If you want to make a C8-looking gun,any base gun would work really if you're not intending to get a receiver with the C8 markings. If you're ok with the stock barrel and whatever markings the receiver comes with, then you're good to go.
Personally, I'd say if you want to go all out and make a genuine looking C8 with the receiver, barrel, etc, I'd just do a build from scratch, because once you get all the parts replaced, there won't be anything left of the base gun other than the internals. It will cost a lot more starting from a base gun than building from scratch.
The major external differences are:
- Receiver with Diemaco / CF markings if doing mil version, Colt Canada markings for Law Enforcement version
- Heavy barrel if doing the A3 variant
- ambi fire selector and mag catch
- tactical charging handle (mil version)
- TRIAD rail
- green furniture (mil version)
- slim hand guards
- ELCAN scope with green cover (although EOTech 552 is commonly used)
- rubber butt pad