For the 51 I would use a JLD receiver. They're high quality and the HK factory parts and accessories fit nicely.
For the 53 you don't have much choice. Vector receivers which are made by Special Weapons/Cohaire are the only ones to be had.
Do you already have all the other parts? Like the trunnion, barrel, bolt head and carrier, yada yada yada? Those things can add up quick. By the time you buy all those parts, it's not too much cheaper than just buying a well-used complete 91 or 93. Actually, in the case of the 51 it might be cheaper to go with the JLD receiver since G3 parts are so readily available.
But if you look around, you can spot a 93 every so often for $2,000, and it will save some money on the build because there's less work to do than if they build it from a blank receiver.
A forum dedicated to and laser-focused on Heckler & Koch firearms, Heckler & Koch accessories and the owners and enthusiasts that love them! Come join the discussion regarding HK pistols, long arms, NFA arms, HK accessories, HK history and trivia and more.