The BuckMark is a spiffy gun, in fact if I had it all to do over again I might have a BuckMark pistol or two and a BuckMark carbine as a companion, sharing mags and an "ultimate cliploader", which in theory works with BuckMark magazines just like it works with Ruger mkII/III magazines.
But after tinkering my mkIII 5.5" bull barrel is about perfect for my hand and trigger preferences, and MrsBFD has hers tweaked in also. I don't think the BuckMark has the aftermarket options that the mkIII does, although it can take a new barrel without ordering a "gun" according to the ATF's asinine opinions on what part of a Ruger .22 pistol is "a gun".
But the used mkIII 22/45 is a steal, maybe can often find some poor schmuck who can't understand the basic care and maintenance of the gun to offload it to you for a good price.