Purpose built VFO's, as you call them, generally allow for presetting an
offset or offsets such that a "dialed-in" frequency reads out directly
in the use frequency rather than the actual mixing or insertion
frequency. Additionally, as with S-Line equipment, etc. that use
band-dependent, multiple mixing formulas, multiple offsets may be set
into VFOs such as the S&S unit I mentioned. Cost wise, they probably
cost the same initially but I'd hate to have to pay to repair an HP. I
personally have not experienced the front-end problems as you describe,
but then I generally use my R-390A's. The 51S1AF/551G sort of sit on
the shelf, looking pretty. I did an article on general-coverage for the
Drake "C" line via a purpose-built VFO in an ER article last year. You
might, perhaps, be interested in having a look at it.
Regards Jeep/K3HVG