Lemme take your questions in reverse order...
The Vector V53 is a clone of the HK53 built on a US made receiver with parts from surplus HK33's and 53's. As far as I'm aware, there are no original factory HK53's in the US that are transferable, only post-sample guns (for licensed dealers and manufacturers only). The non-Vector HK53's you see advertised on places like Subguns and sturmgewehr are made from HK93's which haven't been imported since 1989, thus, the original German HK93 host from which they are built are more rare and because they were built by HK in Germany, they command a premium price. Additionally, there is a significant amount of work to cut down the barrel & cocking tube from the full sized 93 to 53 specs.
A registered sear should work just fine in a V53, but the sear does not just drop-in the trigger pack. Theres a fair amount of gunsmithing that has to be done to th trigger pack to accomodate the registered sear, unless you geta sear thats already been installed into a pack.
