I am at 25% right now after about 3 years.
I buy mostly low grade cards based on eye appeal and have cards with great corners but paperloss on the back or wrinkled corners or cards with well rounded corners but great centering and registration. I am not too picky when it comes to condition as long as the card is in one piece and not stained. I try to find good value.
I have 25 HOFers with a red Cobb and a Young portrait. My approach is to try for one of the major HOFers every year and then pick up a few lower tier HOFers while working toward getting at least one card of every HOFer.
I keep an eye out for different backs but don't really pursue them. I have been very lucky on ebay in the past buying $15 commons without back scans and getting a Peidmont factory 42 back, a Cycle 350 and 460.
I too have a few cards that will need to be replaced some day that came in lots with others.
How ever you go about your set, enjoy the journey.