G3/HK91 build

[megadeth22885]

im thinking of building either an AK or an HK, if i can get all i need to do the HK id prefer that.. i was wondering if anyone knows where a complete source of parts or a parts kit i could use could be found.. and what receiver i should use? im thinking a special weapons receiver, if not a jld.. also, i hear horrible things about the century HKs being put together like garbage.. but is the receiver itself still good for a careful custom build? and whats a cast steel receiver cost?